Intent to ship: @starting-style rule

[Boris Chiou]

As of fx129, I intend to turn `@starting-style` rule on by default on all platforms. It has been developed behind the "layout.css.starting-style-at-rules.enabled" preference.

Status in other browsers: Both Blink and WebKit have shipped this feature.

Bug to turn on by default: https://bugzilla.mozilla.org/show_bug.cgi?id=1900458

Standard: https://drafts.csswg.org/css-transitions-2/#at-ruledef-starting-style

This feature was previously discussed in this "Intent to prototype" thread: https://groups.google.com/a/mozilla.org/g/dev-platform/c/IXyZWh8oJek/m/OPB57VvtAAAJ

Thanks.

Regards,

Boris