LibWeb: Insert WindowProperties object into Window's prototype chain

And implement WindowProperties, the "named properties object" for Window
according to the spec.

This involves moving an AO out of LegacyPlatformObject and into a common
place that the WindowProperties class can access.

This doesn't implement the AOs on Window that actually name lookup for
the unenumerable named properties on the window yet, just the
scaffolding.

Userland/Libraries/LibWeb/HTML/Window.h

@@ -193,6 +193,9 @@ public:
 
     [[nodiscard]] OrderedHashMap<String, JS::NonnullGCPtr<Navigable>> document_tree_child_navigable_target_name_property_set();
 
+    [[nodiscard]] Vector<DeprecatedString> supported_property_names();
+    [[nodiscard]] WebIDL::ExceptionOr<JS::Value> named_item_value(DeprecatedFlyString const&);
+
 private:
     explicit Window(JS::Realm&);