LibWeb: Begin implementing media resource seeking

Author: https://github.com/trflynn89
Commit: 5a98a5529f
Pull-request: https://github.com/SerenityOS/serenity/pull/18296
Reviewed-by: https://github.com/Zaggy1024 ✅

Userland/Libraries/LibWeb/HTML/HTMLMediaElement.idl

@@ -28,6 +28,7 @@ interface HTMLMediaElement : HTMLElement {
     const unsigned short HAVE_FUTURE_DATA = 3;
     const unsigned short HAVE_ENOUGH_DATA = 4;
     readonly attribute unsigned short readyState;
+    readonly attribute boolean seeking;
 
     // playback state
     attribute double currentTime;