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/domstringmap.rs

@@ -67,5 +67,10 @@ impl<'a> DOMStringMapMethods for &'a DOMStringMap {
             }
         }
     }
-}
 
+    // https://html.spec.whatwg.org/multipage/#domstringmap
+    fn SupportedPropertyNames(self) -> Vec<DOMString> {
+        // FIXME: unimplemented (https://github.com/servo/servo/issues/7273)
+        vec![]
+    }
+}