style: Add simple parsing and matching support for :has

Parsing is behind a config value `layout.css.has-selectors.enabled`. This
change does not support p:has(> a) combinators, but will handle them
gracefully, just not matching on them.

Differential Revision: https://phabricator.services.mozilla.com/D149515

components/style/gecko/selector_parser.rs

@@ -305,6 +305,11 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
         true
     }
 
+    #[inline]
+    fn parse_has(&self) -> bool {
+        static_prefs::pref!("layout.css.has-selector.enabled")
+    }
+
     #[inline]
     fn parse_part(&self) -> bool {
         true