LibWeb: Add initial support for CustomEvent

This is used surprisingly often. For example, it is used by a core
YouTube library called Structured Page Fragments.

It allows you to manually dispatch an event with arbitrary data
attached to it.

The only thing missing from this implementation is the constructor.
This is because WrapperGenerator is currently missing dictionary
capabilities.

Userland/Libraries/LibWeb/Forward.h

@@ -62,6 +62,7 @@ class AbortController;
 class AbortSignal;
 class CharacterData;
 class Comment;
+class CustomEvent;
 class Document;
 class DocumentFragment;
 class DocumentLoadEventDelayer;
@@ -261,6 +262,7 @@ class CanvasRenderingContext2DWrapper;
 class CharacterDataWrapper;
 class CloseEventWrapper;
 class CommentWrapper;
+class CustomEventWrapper;
 class DocumentFragmentWrapper;
 class DocumentTypeWrapper;
 class DocumentWrapper;