LibWeb/IDB: Implement IDBObjectStore::index

Author: https://github.com/stelar7
Commit: fce936e05a
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4178
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/AtkinsSJ

Libraries/LibWeb/IndexedDB/IDBObjectStore.h

@@ -41,6 +41,7 @@ public:
 
     WebIDL::ExceptionOr<GC::Ref<IDBIndex>> create_index(String const&, KeyPath, IDBIndexParameters options);
     [[nodiscard]] GC::Ref<HTML::DOMStringList> index_names();
+    WebIDL::ExceptionOr<GC::Ref<IDBIndex>> index(String const&);
 
 protected:
     explicit IDBObjectStore(JS::Realm&, GC::Ref<ObjectStore>, GC::Ref<IDBTransaction>);