[css-fonts-4] Add `hkna` & `vkna` (& `pkna`) OpenType feature settings to `font-variant-east-asian` (e.g. `horizontal-kana` & `vertical-kana` & `proportional-kana`)

[tats-u]

https://helpx.adobe.com/fonts/using/open-type-syntax.html
https://sparanoid.com/lab/opentype-features/
https://moji-memo.hatenablog.jp/entry/20090804/1249369271 (Japanese)
https://lists.w3.org/Archives/Public/public-html-ig-jp/2011Oct/0008.html (Japanese)

Some best-quality (proprietary) Japanese fonts (e.g. Yu Gothic, Hiragino Kaku Gothic/Mincho) supporting Adobe Japan 1-4+ have horizontal/vertical-dedicated hiragana/katakana variants. The default kana glyphs are not fine-tuned for a specific writing direction in such fonts.

[lang]:lang(ja) {
    font: horizontal-kana 100% "Hiragino Sans", "Hiragino Kaku Gothic ProN", "Yu Gothic Medium", sans-serif;
}

ruby {
  font-variant-east-asian: ruby;
}

/* <span class=quote>“</span>ほげ<span class=quote>”</span> */
.quote {
  font-variant-east-asian: full-width;
}

I heard that some fonts contain horizontal/vertical-dedicated kana variants for ruby (Adobe Japan 1-5+?), too.

proportional-width (pwid) is insufficient because it changes widths of unexpected other characters (e.g. "～" equivalent to en-dash in English), too.

Note: pkna for hiragana/katakana = hkna + palt? palt shrinks the widths of some other characters (e.g. full-width parenthesis/brackets), too.

Playground for non-ruby hkna/pkna (Win/mac): https://codepen.io/tats-u/pen/pvbYNoY

[svgeesus]

@kojiishi @xfq @r12a this seems reasonable, comments welcome