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/Bindings/HTMLCollectionWrapperCustom.cpp

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/ScopeGuard.h>
+#include <LibWeb/Bindings/HTMLCollectionWrapper.h>
+#include <LibWeb/Bindings/NodeWrapper.h>
+#include <LibWeb/Bindings/NodeWrapperFactory.h>
+#include <LibWeb/DOM/Element.h>
+
+namespace Web::Bindings {
+
+JS::Value HTMLCollectionWrapper::get(JS::PropertyName const& name, JS::Value receiver, bool without_side_effects) const
+{
+    auto* item = const_cast<DOM::HTMLCollection&>(impl()).named_item(name.to_string());
+    if (!item)
+        return Base::get(name, receiver, without_side_effects);
+    return JS::Value { wrap(global_object(), *item) };
+}
+
+JS::Value HTMLCollectionWrapper::get_by_index(u32 property_index) const
+{
+    auto* item = const_cast<DOM::HTMLCollection&>(impl()).item(property_index);
+    if (!item)
+        return Base::get_by_index(property_index);
+    return wrap(global_object(), *item);
+}
+
+}