LibWeb: Implement AbortSignal.onabort

Author: https://github.com/Lubrsi
Commit: 971d60c329
Pull-request: https://github.com/SerenityOS/serenity/pull/10285

Userland/Libraries/LibWeb/DOM/AbortSignal.h

@@ -46,6 +46,9 @@ public:
 
     void signal_abort();
 
+    void set_onabort(HTML::EventHandler);
+    HTML::EventHandler onabort();
+
     // ^EventTarget
     virtual void ref_event_target() override { ref(); }
     virtual void unref_event_target() override { unref(); }