LibWeb: Implement DOMTokenList for managing space-separated tokens lists

DOMTokenList is used as the return type of, e.g., the Element.classList
property.

Userland/Libraries/LibWeb/Forward.h

@@ -81,6 +81,7 @@ class DocumentLoadEventDelayer;
 class DocumentType;
 class DOMException;
 class DOMImplementation;
+class DOMTokenList;
 class Element;
 class Event;
 class EventHandler;
@@ -313,6 +314,7 @@ class DOMParserWrapper;
 class DOMRectReadOnlyWrapper;
 class DOMRectWrapper;
 class DOMStringMapWrapper;
+class DOMTokenListWrapper;
 class ElementWrapper;
 class EventListenerWrapper;
 class EventTargetWrapper;