Audit of Japanese fonts

Some of our Japanese fonts display kana-only (which might be OK because they are kana only fonts, but also might indicate a problem with kanji coverage). Some of them display Latin sample text. I'm proposing to restrict the `ja_Jpan` exemplar kanji to [Joyo kanji only](https://github.com/googlefonts/lang/pull/130), but I'm going to use this issue to document whether or not that would actually fix the problem.

* [Noto Sans Japanese](https://fonts.google.com/noto/specimen/Noto+Sans+JP) shows a kana-only specimen when the `subset` filter is set to Japanese. This is wrong, and I'm not sure why this is the case especially given that [Noto Serif Japanese](https://fonts.google.com/noto/specimen/Noto+Serif+JP) does not. METADATA.pb languages and primary script are the same in both fonts.
* Display fonts. The following fonts show kana-only in the samples, but they are kana-only fonts so that's *fine*:
* [Shizuru](https://fonts.google.com/specimen/Shizuru)
* [Aoboshi One](https://fonts.google.com/specimen/Aoboshi+One?subset=japanese)
* [Monomaniac One](https://fonts.google.com/specimen/Monomaniac+One)
* [Tsukimi Rounded](https://fonts.google.com/specimen/Tsukimi+Rounded?subset=japanese¬o.script=Hira)
* [Cherry Bomb One](https://fonts.google.com/specimen/Cherry+Bomb+One?subset=japanese¬o.script=Hira)
* [Chokokutai](https://fonts.google.com/specimen/Chokokutai?subset=japanese¬o.script=Hira)
* [Darumadrop One](https://fonts.google.com/specimen/Darumadrop+One?subset=japanese¬o.script=Hira)
* [Slackside One](https://fonts.google.com/specimen/Slackside+One?subset=japanese¬o.script=Hira)
* [Palette Mosaic](https://fonts.google.com/specimen/Palette+Mosaic?subset=japanese¬o.script=Hira)
* [Yuji Hentaigana Akari](https://fonts.google.com/specimen/Yuji+Hentaigana+Akari?subset=japanese¬o.script=Hira) (Specialist, non-Unicode font)
* [Yuji Hentaigana Akebono](https://fonts.google.com/specimen/Yuji+Hentaigana+Akebono?subset=japanese¬o.script=Hira) (Specialist, non-Unicode font)
* [Rock 3D](https://fonts.google.com/specimen/Rock+3D?subset=japanese¬o.script=Hira)
* The following fonts show a Latin preview on main fgc page, show a Latin specimen if `subset=japanese` is not in the URL and a Japanese specimens if it is. They probably need a primary-script metadata field. Most of these fonts support all of Joyo kanji *except* the three Joyo kanji: 剝, 塡, 頰. These kanji are kind of outliers anyway. They are 2010 additions to the Joyo list, they are not part of the JIS character set, and are usually replaced by 填, 剥, and 頬, but they're in there now. Maybe we want to take these out of the exemplars.
* [M PLUS Rounded 1c](https://fonts.google.com/specimen/M+PLUS+Rounded+1c). The odd thing about this font is that I can't find out where it lives in the repo. `git grep -i "M PLUS Rounded 1c"` produces no results at all. Weird, weird, weird.
* [Potta One](https://fonts.google.com/specimen/Potta+One) (No 填剥頬)
* [Zen Antique](https://fonts.google.com/specimen/Zen+Antique) (No 填剥頬)
* [Kosugi Maru](https://fonts.google.com/specimen/Kosugi+Maru) (No 填剥頬)
* [Sawarabi Gothic](https://fonts.google.com/specimen/Sawarabi+Gothic) (full set)
* [Shippori Mincho](https://fonts.google.com/specimen/Shippori+Mincho) (full set)
* [Shippori Antique](https://fonts.google.com/specimen/Shippori+Antique) (full set)
* [Shippori Antique B1](https://fonts.google.com/specimen/Shippori+Antique+B1) (full set)
* [Kiwi Maru](https://fonts.google.com/specimen/Kiwi+Maru) (full set)
* [Zen Old Mincho](https://fonts.google.com/specimen/Zen+Old+Mincho?subset=japanese¬o.script=Hira) (No 填剥頬)
* [Zen Antique Soft](https://fonts.google.com/specimen/Zen+Antique+Soft?subset=japanese¬o.script=Hira) (No 填剥頬)
* [Kaisei Decol](https://fonts.google.com/specimen/Kaisei+Decol?subset=japanese¬o.script=Hira) (full set)
* [Kaisei Opti](https://fonts.google.com/specimen/Kaisei+Opti?subset=japanese¬o.script=Hira) (full set)
* [Klee One](https://fonts.google.com/specimen/Klee+One?subset=japanese¬o.script=Hira) (full set)
* [RocknRoll One](https://fonts.google.com/specimen/RocknRoll+One?subset=japanese¬o.script=Hira) (full set)
* [BIZ UDP Mincho](https://fonts.google.com/specimen/BIZ+UDPMincho)
* [BIZ UD Mincho](https://fonts.google.com/specimen/BIZ+UDMincho)
* [Kaisei Tokumin](https://fonts.google.com/specimen/Kaisei+Tokumin)
* [Kaisei HarunoUmi](https://fonts.google.com/specimen/Kaisei+HarunoUmi)
* [Yuji Syuku](https://fonts.google.com/specimen/Yuji+Syuku)
* [Yuji Mai](https://fonts.google.com/specimen/Yuji+Mai)
* [Yuji Boku](https://fonts.google.com/specimen/Yuji+Boku)
* [New Tegomin](https://fonts.google.com/specimen/New+Tegomin) (No 填剥頬)
* The following fonts show *kana* in the specimen page but they do have a full Joyo kanji set:
* [M PLUS 1p](https://fonts.google.com/specimen/M+PLUS+1p?subset=japanese)
* [M PLUS 1](https://fonts.google.com/specimen/M+PLUS+1?subset=japanese)
* [M PLUS 1 Code](https://fonts.google.com/specimen/M+PLUS+1+Code?subset=japanese)
* [M PLUS 2](https://fonts.google.com/specimen/M+PLUS+2?subset=japanese)
* [Dela Gothic One](https://fonts.google.com/specimen/Dela+Gothic+One)
* [BIZ UDP Gothic](https://fonts.google.com/specimen/BIZ+UDPGothic)
* [BIZ UD Gothic](https://fonts.google.com/specimen/BIZ+UDGothic)
* [Zen Kaku Gothic New](https://fonts.google.com/specimen/Zen+Kaku+Gothic+New)
* [Zen Maru Gothic](https://fonts.google.com/specimen/Zen+Maru+Gothic) except it is missing 填, 剥, and 頬.
* [Zen Kaku Gothic Antique](https://fonts.google.com/specimen/Zen+Kaku+Gothic+Antique) (No 填剥頬)
* [Kosugi](https://fonts.google.com/specimen/Kosugi) (No 填剥頬)
* [Murecho](https://fonts.google.com/specimen/Murecho) (No 叱, which is another of the "funny" new Joyo kanji)
* [Shippori Mincho B1](https://fonts.google.com/specimen/Shippori+Mincho+B1) (full set)
* [DotGothic16](https://fonts.google.com/specimen/DotGothic16?subset=japanese¬o.script=Hira) (full set)
* [Zen Kurenaido](https://fonts.google.com/specimen/Zen+Kurenaido?subset=japanese¬o.script=Hira) (No 填剥頬)
* [Yusei Magic](https://fonts.google.com/specimen/Yusei+Magic?subset=japanese¬o.script=Hira)
* [Mochiy Pop One](https://fonts.google.com/specimen/Mochiy+Pop+One?subset=japanese¬o.script=Hira)
* [Mochiy Pop P One](https://fonts.google.com/specimen/Mochiy+Pop+P+One?subset=japanese¬o.script=Hira)
* [Reggae One](https://fonts.google.com/specimen/Reggae+One?subset=japanese¬o.script=Hira)
* [Hina Mincho](https://fonts.google.com/specimen/Hina+Mincho?subset=japanese¬o.script=Hira)
* [Rampart One](https://fonts.google.com/specimen/Rampart+One?subset=japanese¬o.script=Hira)
* [Hachi Mari Pop](https://fonts.google.com/specimen/Hachi+Mari+Pop) (No 填剥頬)
* [Stick](https://fonts.google.com/specimen/Stick) (full set)
* [IBM Plex Sans JP](https://fonts.google.com/specimen/IBM+Plex+Sans+JP) (full set)
* [Yomogi](https://fonts.google.com/specimen/Yomogi) (full set except 々, which really should be added!)
* [Sawarabi Mincho](https://fonts.google.com/specimen/Sawarabi+Mincho?subset=japanese¬o.script=Hira) is missing quite a lot of Joyo, including some fairly key ones. You can't really use it for Japanese running text. It should be fixed. I don't know if we should make it a display font in the interim - it's not very display-y.
* [Train One](https://fonts.google.com/specimen/Train+One?subset=japanese¬o.script=Hira) is missing a small number (32) of fairly rare Joyo. Can we commission a small expansion? It's currently showing Latin in the fgc page and should probably show kana until fixed.