Intent to ship: CSS font-variant-emoji

[Jonathan Kew]

As of Firefox 141, I intend to turn the CSS property
`font-variant-emoji` on by default for all channels.

It has been developed behind the `layout.css.font-variant-emoji.enabled
preference` at https://bugzilla.mozilla.org/show_bug.cgi?id=1461589

Status in other browsers is:

    Chrome: supported since v131
    Safari: currently behind an experimental-feature flag

Bug to turn on by default:
https://bugzilla.mozilla.org/show_bug.cgi?id=1954214

Standard: https://drafts.csswg.org/css-fonts-4/#font-variant-emoji-prop

- JK