LibWeb/IDB: Implement IDBCursorWithValue interface

Author: https://github.com/stelar7
Commit: 296d9d74d8
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4720
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/AtkinsSJ ✅
Reviewed-by: https://github.com/shannonbooth

Libraries/LibWeb/Forward.h

@@ -624,6 +624,7 @@ class Performance;
 namespace Web::IndexedDB {
 class Database;
 class IDBCursor;
+class IDBCursorWithValue;
 class IDBDatabase;
 class IDBFactory;
 class IDBIndex;