LibWeb/HTML: Implement TextTrackCue idl interface

Author: https://github.com/jamierocks
Commit: 0b2449d8d2
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1494
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/tcl3 ✅

Userland/Libraries/LibWeb/Forward.h

@@ -496,6 +496,7 @@ class Storage;
 class SubmitEvent;
 class TextMetrics;
 class TextTrack;
+class TextTrackCue;
 class TextTrackList;
 class Timer;
 class TimeRanges;