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/style/invalidation/element/invalidation_map.rs

@@ -98,6 +98,7 @@ impl Dependency {
             // an eager pseudo, and return only Descendants here if not.
             Some(Combinator::PseudoElement) => DependencyInvalidationKind::ElementAndDescendants,
             Some(Combinator::SlotAssignment) => DependencyInvalidationKind::SlottedElements,
+            Some(Combinator::Part) => unimplemented!("Need to add invalidation for shadow parts"),
         }
     }
 }