Intent to prototype & ship: CSS :first (first-page pseudo-class)

[Emily McDonough]

As of Firefox 116, the :first pseudo-class for @page rules has been implemented and enabled by default.

`:first` allows a page-rule to match only the first printed page.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1833471
Standard: https://drafts.csswg.org/css-page-3/#first-pseudo
Testing: Firefox-specific tests included with the patch, subsequently merged into WPT tests in https://bugzilla.mozilla.org/show_bug.cgi?id=1787073
Other browsers: Already implemented in Blink and Webkit
Platform Coverage: All
Web platform tests: Some present in https://wpt.fyi/results/css/css-page and some in https://wpt.fyi/results/css/printing