LibWeb: Implement a slow but functional HTMLCollection :^)

HTMLCollection is an awkward legacy interface from the DOM spec.

It provides a live view of a DOM subtree, with some kind of filtering
that determines which elements are part of the collection.

We now return HTMLCollection objects from these APIs:

- getElementsByClassName()
- getElementsByName()
- getElementsByTagName()

This initial implementation does not do any kind of caching, since that
is quite a tricky problem, and there will be plenty of time for tricky
problems later on when the engine is more mature.

Userland/Libraries/LibWeb/Forward.h

@@ -42,6 +42,7 @@ class Event;
 class EventHandler;
 class EventListener;
 class EventTarget;
+class HTMLCollection;
 class MouseEvent;
 class Node;
 class ParentNode;
@@ -218,6 +219,7 @@ class HTMLBodyElementWrapper;
 class HTMLBRElementWrapper;
 class HTMLButtonElementWrapper;
 class HTMLCanvasElementWrapper;
+class HTMLCollectionWrapper;
 class HTMLDataElementWrapper;
 class HTMLDataListElementWrapper;
 class HTMLDetailsElementWrapper;