LibWeb: Implement ParentNode.prepend

`convert_nodes_to_single_node` is inside its own file so ChildNode can
include and use it without having to include other headers such as
DOM/Node.h. This is to prevent circular includes.

Userland/Libraries/LibWeb/DOM/NodeOperations.h

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2022, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::DOM {
+
+ExceptionOr<NonnullRefPtr<Node>> convert_nodes_to_single_node(Vector<Variant<NonnullRefPtr<Node>, String>> const& nodes, DOM::Document& document);
+
+}