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.cpp

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2022, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/String.h>
+#include <AK/Vector.h>
+#include <LibWeb/DOM/DocumentFragment.h>
+#include <LibWeb/DOM/NodeOperations.h>
+#include <LibWeb/DOM/Text.h>
+
+namespace Web::DOM {
+
+// https://dom.spec.whatwg.org/#converting-nodes-into-a-node
+ExceptionOr<NonnullRefPtr<Node>> convert_nodes_to_single_node(Vector<Variant<NonnullRefPtr<Node>, String>> const& nodes, DOM::Document& document)
+{
+    // 1. Let node be null.
+    // 2. Replace each string in nodes with a new Text node whose data is the string and node document is document.
+    // 3. If nodes contains one node, then set node to nodes[0].
+    // 4. Otherwise, set node to a new DocumentFragment node whose node document is document, and then append each node in nodes, if any, to it.
+    // 5. Return node.
+
+    auto potentially_convert_string_to_text_node = [&document](Variant<NonnullRefPtr<Node>, String> const& node) -> NonnullRefPtr<Node> {
+        if (node.has<NonnullRefPtr<Node>>())
+            return node.get<NonnullRefPtr<Node>>();
+
+        return adopt_ref(*new Text(document, node.get<String>()));
+    };
+
+    if (nodes.size() == 1)
+        return potentially_convert_string_to_text_node(nodes.first());
+
+    // This is NNRP<Node> instead of NNRP<DocumentFragment> to be compatible with the return type.
+    NonnullRefPtr<Node> document_fragment = adopt_ref(*new DocumentFragment(document));
+    for (auto& unconverted_node : nodes) {
+        auto node = potentially_convert_string_to_text_node(unconverted_node);
+        auto result = document_fragment->append_child(node);
+        if (result.is_exception())
+            return result.exception();
+    }
+
+    return document_fragment;
+}
+
+}