style: Implement :-moz-any() as an alias of :is().

This is strictly better and more flexible, but can change specificity so
have a pref in case it causes trouble. I doubt it will though, the
specificity rules of :is() make more sense, and my gut feeling is that
:-moz-any is not very used on the wild.

Make it early-beta-or-earlier for now to minimize risk, once this is on
nightly for a bit we can enable it everywhere.

Differential Revision: https://phabricator.services.mozilla.com/D86696

components/style/gecko/selector_parser.rs

@@ -352,6 +352,12 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
         true
     }
 
+    #[inline]
+    fn is_is_alias(&self, function: &str) -> bool {
+        static_prefs::pref!("layout.css.moz-any-is-is.enabled") &&
+            function.eq_ignore_ascii_case("-moz-any")
+    }
+
     fn parse_non_ts_pseudo_class(
         &self,
         location: SourceLocation,