LibWeb: Add stub implementation of HTMLOptionsCollection

This is a subtype of `DOM::HTMLCollection` that only holds
`HTMLOptionElement`s. In this stub implementation only `item`,
`namedItem` and `length`, inherited from HTMLCollection, are exposed.
This is good enough for applications that only read the collection.

Userland/Libraries/LibWeb/Forward.h

@@ -199,6 +199,7 @@ class HTMLObjectElement;
 class HTMLOListElement;
 class HTMLOptGroupElement;
 class HTMLOptionElement;
+class HTMLOptionsCollection;
 class HTMLOutputElement;
 class HTMLParagraphElement;
 class HTMLParamElement;
@@ -446,6 +447,7 @@ class HTMLObjectElementWrapper;
 class HTMLOListElementWrapper;
 class HTMLOptGroupElementWrapper;
 class HTMLOptionElementWrapper;
+class HTMLOptionsCollectionWrapper;
 class HTMLOutputElementWrapper;
 class HTMLParagraphElementWrapper;
 class HTMLParamElementWrapper;