Support `@font-feature-values`

[yisibl]

@font-feature-values Inter {
    @character-variant {
        cv05: 5;
        cv08: 8;
        cv10: 10;
    }
    @styleset {
        ss01: 1;
        ss08: 8;
    }
}

• Spec: https://drafts.csswg.org/css-fonts/#font-feature-values
• MDN: https://developer.mozilla.org/en-US/docs/Web/CSS/@font-feature-value
• Demo: https://x.com/yisibl/status/1846557957896765763

[yisibl]

Test case:

• https://github.com/clagnut/TODS/blob/e693d52ad411507b960cf01a9734265e3efab102/tods.css#L116-L142
• https://github.com/Sorixelle/srxl.me/blob/4eb4f4a15cb2d21356df24c096d6a819cfdc1a99/public/fonts/inter/inter.css#L201-L222
• https://github.com/MihailJP/Inconsolata-LGC/blob/7c53cf455787096c93d82d9a51018f12ec39a6e9/Inconsolata-LGC.css#L65-L91

[yisibl]

Also, following the principle of retaining only the last-defined value, as well as the principle of shortest representation, multiply-specified tuples and multiple blocks are serialized as a single block containing only the last-defined value.

For example, the declaration: https://drafts.csswg.org/css-fonts/#ex-serialize-same-font-feature-values

/* Repeated declaration names, and multiple blocks of the same type*/
@font-feature-values foo {
  @swash { pretty: 0; cool: 2; }
  @swash { pretty: 1; }
}

would be serialized as:

/* Canonical serialization */
@font-feature-values foo {
    @swash { cool: 2; pretty: 1; }
}

[yisibl]

Note that when compressing, we also need to handle special versions of Firefox that don't remove the ending semicolon.

@font-feature-values FontName {
  font-display: whatever;
}

https://x.com/pjchrobak/status/1059229076987301888