Intent to ship: `size-adjust` descriptor, extended `font-size-adjust` syntax

[Jonathan Kew]

As of Firefox 92, I intend to turn the CSS `size-adjust` descriptor and
extended syntax of the `font-size-adjust` property on by default.

`size-adjust` was developed behind the `layout.css.size-adjust.enabled`
pref in bug 1698495.

The extended `font-size-adjust` syntax was developed behind the
`layout.css.font-size-adjust.basis.enabled` pref in bug 1711479 and bug
1716841.

Status in other browsers:
* Blink:
- `size-adjust` intent to ship:
https://groups.google.com/a/chromium.org/g/blink-dev/c/1PVr94hZHjU
- `font-size-adjust` is currently implemented behind a testing flag,
though not yet updated to the newly-extended syntax
* WebKit: no public signal re implementation, but participated
positively in the spec development

Spec links:
https://drafts.csswg.org/css-fonts-5/#size-adjust-desc
https://drafts.csswg.org/css-fonts-5/#font-size-adjust-prop

Bug to turn on by default:
https://bugzilla.mozilla.org/show_bug.cgi?id=1720131

The `size-adjust` feature was previously discussed in this "Intent to
prototype" thread:
https://groups.google.com/g/mozilla.dev.platform/c/joBc4SoTOPc

(`font-size-adjust` is not a new feature, but extension of the values
accepted by an existing property. But given that these two features, the
`size-adjust` descriptor for `@font-face` and the `font-size-adjust`
property, are designed for similar purposes and in fact interact with
each other -- in that when `font-size-adjust` is used, it effectively
overrides the result of any `size-adjust` descriptors -- I think it
makes sense to treat them together for the purpose of shipping.)