LibWeb: Add HTMLOptionsCollection selected index property

Author: https://github.com/bplaat
Commit: 5decf4b33c
Pull-request: https://github.com/SerenityOS/serenity/pull/23895
Reviewed-by: https://github.com/shannonbooth

Userland/Libraries/LibWeb/HTML/HTMLOptionsCollection.h

@@ -28,6 +28,9 @@ public:
 
     WebIDL::ExceptionOr<void> add(HTMLOptionOrOptGroupElement element, Optional<HTMLElementOrElementIndex> before = {});
 
+    WebIDL::Long selected_index() const;
+    void set_selected_index(WebIDL::Long);
+
 private:
     HTMLOptionsCollection(DOM::ParentNode& root, Function<bool(DOM::Element const&)> filter);