Initial implementation of ownPropertyKeys proxy handler

Generates `SupportedPropertyNames` on DOM structs that should implement
it. Most of them are unimplemented now (which can be implemented in
later PRs), with the exception of `HTMLCollection`. Also added a couple
relevant WPT tests.

Closes #6390

Closes #2215

components/script/dom/namednodemap.rs

@@ -105,5 +105,9 @@ impl<'a> NamedNodeMapMethods for &'a NamedNodeMap {
         *found = item.is_some();
         item
     }
-}
 
+    fn SupportedPropertyNames(self) -> Vec<DOMString> {
+        // FIXME: unimplemented (https://github.com/servo/servo/issues/7273)
+        vec![]
+    }
+}