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/selectors/parser.rs

@@ -254,6 +254,11 @@ pub trait Parser<'i> {
         false
     }
 
+    /// Whether the given function name is an alias for the `:is()` function.
+    fn is_is_alias(&self, _name: &str) -> bool {
+        false
+    }
+
     /// Whether to parse the `:host` pseudo-class.
     fn parse_host(&self) -> bool {
         false
@@ -2302,6 +2307,10 @@ where
         _ => {}
     }
 
+    if parser.parse_is_and_where() && parser.is_is_alias(&name) {
+        return parse_is_or_where(parser, input, state, Component::Is);
+    }
+
     if !state.allows_custom_functional_pseudo_classes() {
         return Err(input.new_custom_error(SelectorParseErrorKind::InvalidState));
     }