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/HTML/HTMLOptionsCollection.idl

@@ -0,0 +1,12 @@
+#import <DOM/HTMLCollection.idl>
+#import <HTML/HTMLOptionElement.idl>
+#import <HTML/HTMLOptGroupElement.idl>
+
+[Exposed=Window]
+interface HTMLOptionsCollection : HTMLCollection {
+    // [CEReactions] attribute unsigned long length; // shadows inherited length
+    // [CEReactions] setter undefined (unsigned long index, HTMLOptionElement? option);
+    // [CEReactions] undefined add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
+    // [CEReactions] undefined remove(long index);
+    // attribute long selectedIndex;
+};