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/CMakeLists.txt

@@ -3,6 +3,7 @@ set(SOURCES
     Bindings/EventListenerWrapper.cpp
     Bindings/EventWrapperFactory.cpp
     Bindings/EventTargetWrapperFactory.cpp
+    Bindings/HTMLCollectionWrapperCustom.cpp
     Bindings/ImageConstructor.cpp
     Bindings/LocationObject.cpp
     Bindings/MainThreadVM.cpp
@@ -48,6 +49,7 @@ set(SOURCES
     DOM/Element.cpp
     DOM/ElementFactory.cpp
     DOM/Event.cpp
+    DOM/HTMLCollection.cpp
     DOM/Range.cpp
     DOM/EventDispatcher.cpp
     DOM/EventListener.cpp
@@ -309,6 +311,7 @@ libweb_js_wrapper(DOM/DOMImplementation)
 libweb_js_wrapper(DOM/Element)
 libweb_js_wrapper(DOM/Event)
 libweb_js_wrapper(DOM/EventTarget)
+libweb_js_wrapper(DOM/HTMLCollection)
 libweb_js_wrapper(DOM/ProcessingInstruction)
 libweb_js_wrapper(DOM/ShadowRoot)
 libweb_js_wrapper(DOM/Node)