Missing codepoints compared to dummy-index's font

The following 69 codepoints exist in the font from @dummy-index font but are absent from `xkcd-script.otf` (link to where @dummy-index mentions this: https://github.com/ipython/xkcd-font/issues/11#issuecomment-739486365).

### Punctuation / symbols (standalone)

| Codepoint | Char | Name |
|-----------|------|------|
| U+00A1 | ¡ | INVERTED EXCLAMATION MARK |
| U+00A8 | ¨ | DIAERESIS |
| U+00AB | « | LEFT-POINTING DOUBLE ANGLE QUOTATION MARK |
| U+00AF | ¯ | MACRON |
| U+00B0 | ° | DEGREE SIGN |
| U+00B7 | · | MIDDLE DOT |
| U+00B8 | ¸ | CEDILLA |
| U+00BB | » | RIGHT-POINTING DOUBLE ANGLE QUOTATION MARK |
| U+00BF | ¿ | INVERTED QUESTION MARK |
| U+00D7 | × | MULTIPLICATION SIGN |
| U+2010 | ‐ | HYPHEN |
| U+2036 | ‶ | REVERSED DOUBLE PRIME |

### Latin ligatures / special letters

Done in #72 and #74.

| Codepoint | Char | Name |
|-----------|------|------|
| U+00C6 | Æ | LATIN CAPITAL LETTER AE |
| U+00E6 | æ | LATIN SMALL LETTER AE |
| U+0152 | Π| LATIN CAPITAL LIGATURE OE |
| U+0153 | œ | LATIN SMALL LIGATURE OE |
| U+017F | ſ | LATIN SMALL LETTER LONG S |
| U+0237 | ȷ | LATIN SMALL LETTER DOTLESS J |

### Spacing modifier letters (standalone diacritic forms)

| Codepoint | Char | Name |
|-----------|------|------|
| U+02BB | ʻ | MODIFIER LETTER TURNED COMMA |
| U+02BC | ʼ | MODIFIER LETTER APOSTROPHE |
| U+02C6 | ˆ | MODIFIER LETTER CIRCUMFLEX ACCENT |
| U+02C7 | ˇ | CARON |
| U+02D8 | ˘ | BREVE |
| U+02D9 | ˙ | DOT ABOVE |
| U+02DB | ˛ | OGONEK |
| U+02DC | ˜ | SMALL TILDE |
| U+02DD | ˝ | DOUBLE ACUTE ACCENT |

### Combining diacritics (missing)

| Codepoint | Name |
|-----------|------|
| U+0312 | COMBINING TURNED COMMA ABOVE |
| U+0315 | COMBINING COMMA ABOVE RIGHT |
| U+0326 | COMBINING COMMA BELOW |
| U+0327 | COMBINING CEDILLA |
| U+0328 | COMBINING OGONEK |
| U+0337 | COMBINING SHORT SOLIDUS OVERLAY |
| U+0338 | COMBINING LONG SOLIDUS OVERLAY |

### Precomposed composites (U+01xx block)

Most of these are combinations of base letters we already have; they could potentially be auto-generated via FontForge's `buildComposit` once the base glyphs and marks are in place.

| Codepoint | Char | Name |
|-----------|------|------|
| U+01C4 | DŽ | LATIN CAPITAL LETTER DZ WITH CARON |
| U+01C5 | Dž | LATIN CAPITAL LETTER D WITH SMALL LETTER Z WITH CARON |
| U+01C6 | dž | LATIN SMALL LETTER DZ WITH CARON |
| U+01D5 | Ǖ | LATIN CAPITAL LETTER U WITH DIAERESIS AND MACRON |
| U+01D6 | ǖ | LATIN SMALL LETTER U WITH DIAERESIS AND MACRON |
| U+01D7 | Ǘ | LATIN CAPITAL LETTER U WITH DIAERESIS AND ACUTE |
| U+01D8 | ǘ | LATIN SMALL LETTER U WITH DIAERESIS AND ACUTE |
| U+01D9 | Ǚ | LATIN CAPITAL LETTER U WITH DIAERESIS AND CARON |
| U+01DA | ǚ | LATIN SMALL LETTER U WITH DIAERESIS AND CARON |
| U+01DB | Ǜ | LATIN CAPITAL LETTER U WITH DIAERESIS AND GRAVE |
| U+01DC | ǜ | LATIN SMALL LETTER U WITH DIAERESIS AND GRAVE |
| U+01DE | Ǟ | LATIN CAPITAL LETTER A WITH DIAERESIS AND MACRON |
| U+01DF | ǟ | LATIN SMALL LETTER A WITH DIAERESIS AND MACRON |
| U+01E2 | Ǣ | LATIN CAPITAL LETTER AE WITH MACRON |
| U+01E3 | ǣ | LATIN SMALL LETTER AE WITH MACRON |
| U+01E6 | Ǧ | LATIN CAPITAL LETTER G WITH CARON |
| U+01E7 | ǧ | LATIN SMALL LETTER G WITH CARON |
| U+01E8 | Ǩ | LATIN CAPITAL LETTER K WITH CARON |
| U+01E9 | ǩ | LATIN SMALL LETTER K WITH CARON |
| U+01EA | Ǫ | LATIN CAPITAL LETTER O WITH OGONEK |
| U+01EB | ǫ | LATIN SMALL LETTER O WITH OGONEK |
| U+01EC | Ǭ | LATIN CAPITAL LETTER O WITH OGONEK AND MACRON |
| U+01ED | ǭ | LATIN SMALL LETTER O WITH OGONEK AND MACRON |
| U+01F0 | ǰ | LATIN SMALL LETTER J WITH CARON |
| U+01F1 | DZ | LATIN CAPITAL LETTER DZ |
| U+01F2 | Dz | LATIN CAPITAL LETTER D WITH SMALL LETTER Z |
| U+01F3 | dz | LATIN SMALL LETTER DZ |
| U+01F4 | Ǵ | LATIN CAPITAL LETTER G WITH ACUTE |
| U+01F5 | ǵ | LATIN SMALL LETTER G WITH ACUTE |
| U+01FA | Ǻ | LATIN CAPITAL LETTER A WITH RING ABOVE AND ACUTE |
| U+01FB | ǻ | LATIN SMALL LETTER A WITH RING ABOVE AND ACUTE |
| U+01FC | Ǽ | LATIN CAPITAL LETTER AE WITH ACUTE |
| U+01FD | ǽ | LATIN SMALL LETTER AE WITH ACUTE |
| U+01FE | Ǿ | LATIN CAPITAL LETTER O WITH STROKE AND ACUTE |
| U+01FF | ǿ | LATIN SMALL LETTER O WITH STROKE AND ACUTE |