LibWeb: Implement UserActivation

Author: https://github.com/jamierocks
Commit: 01bd179eef
Pull-request: https://github.com/SerenityOS/serenity/pull/24437
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/mattco98
Reviewed-by: https://github.com/tcl3

Userland/Libraries/LibWeb/Forward.h

@@ -466,6 +466,7 @@ class ToggleEvent;
 class TrackEvent;
 struct TransferDataHolder;
 class TraversableNavigable;
+class UserActivation;
 class VideoTrack;
 class VideoTrackList;
 class Window;