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/DOM/HTMLCollection.idl

@@ -0,0 +1,8 @@
+[CustomGet,CustomGetByIndex]
+interface HTMLCollection {
+
+    readonly attribute unsigned long length;
+    Element? item(unsigned long index);
+    Element? namedItem(DOMString name);
+
+};