LibWeb/IDB: Add internal Index object

Author: https://github.com/stelar7
Commit: 9321ad04c0
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4178
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/AtkinsSJ

Libraries/LibWeb/Forward.h

@@ -625,6 +625,7 @@ class IDBOpenDBRequest;
 class IDBRequest;
 class IDBTransaction;
 class IDBVersionChangeEvent;
+class Index;
 class ObjectStore;
 class RequestList;
 }