LibWeb/IDB: Add internal ObjectStore type

Author: https://github.com/stelar7
Commit: 3c5578cc87
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4077
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/gmta ✅

Libraries/LibWeb/Forward.h

@@ -620,6 +620,7 @@ class IDBOpenDBRequest;
 class IDBRequest;
 class IDBTransaction;
 class IDBVersionChangeEvent;
+class ObjectStore;
 class RequestList;
 }