style: Add parsing support for ::part().

Disabled for now of course. This should be pretty uncontroversial I'd think.

Differential Revision: https://phabricator.services.mozilla.com/D28060

components/selectors/tree.rs

@@ -110,6 +110,11 @@ pub trait Element: Sized + Clone + Debug {
         case_sensitivity: CaseSensitivity,
     ) -> bool;
 
+    fn is_part(
+        &self,
+        name: &<Self::Impl as SelectorImpl>::PartName,
+    ) -> bool;
+
     /// Returns whether this element matches `:empty`.
     ///
     /// That is, whether it does not contain any child element or any non-zero-length text node.