LibWeb: Add support for NodeList

This introduces 3 classes: NodeList, StaticNodeList and LiveNodeList.
NodeList is the base of the static and live versions. Static is a
snapshot whereas live acts on the underlying data and thus inhibits
the same issues we have currently with HTMLCollection.

They were split into separate classes to not have them weirdly
mis-mashed together.

The create functions for static and live both return a NNRP to the base
class. This is to prevent having to do awkward casting at creation
and/or return, as the bindings expect to see the base NodeList only.

Userland/Libraries/LibWeb/DOM/NodeList.h

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Noncopyable.h>
+#include <AK/RefCounted.h>
+#include <LibWeb/Bindings/Wrappable.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::DOM {
+
+// https://dom.spec.whatwg.org/#nodelist
+class NodeList
+    : public RefCounted<NodeList>
+    , public Bindings::Wrappable {
+    AK_MAKE_NONCOPYABLE(NodeList);
+    AK_MAKE_NONMOVABLE(NodeList);
+
+public:
+    using WrapperType = Bindings::NodeListWrapper;
+
+    virtual ~NodeList() override = default;
+
+    virtual u32 length() const = 0;
+    virtual Node const* item(u32 index) const = 0;
+
+    virtual bool is_supported_property_index(u32) const = 0;
+
+protected:
+    NodeList() = default;
+};
+
+}