primary script tool: add "Hira" for kana only fonts

In https://github.com/google/fonts/pull/6826, the primary script module in Fontbakery advises for primary script "Hani". Although "Hani" doesn't work, japanese fonts should have the "Jpan" tag. And the one that doesn't support kanji characters should have the tag "Hira".

⚠ WARN: METADATA.pb: Check for primary_script ([com.google.fonts/check/metadata/primary_script](https://font-bakery.readthedocs.io/en/stable/fontbakery/profiles/googlefonts.html#com.google.fonts/check/metadata/primary_script))
⚠ WARN METADATA.pb: primary_script is 'Hira'
It should instead be 'Hani'. [code: wrong-primary-script]