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.cpp

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/HTML/HTMLOptionsCollection.h>
+
+namespace Web::HTML {
+
+HTMLOptionsCollection::HTMLOptionsCollection(DOM::ParentNode& root, Function<bool(DOM::Element const&)> filter)
+    : DOM::HTMLCollection(root, move(filter))
+{
+}
+
+}