LibWebView+WebContent: Add APIs to manipulate DOM nodes

This adds APIs to allow Ispector clients to:
* Change a DOM text or comment node's text data.
* Add, replace, or remove a DOM element's attribute.
* Change a DOM element's tag.

Userland/Libraries/LibWebView/Attribute.h

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/String.h>
+#include <LibIPC/Forward.h>
+
+namespace WebView {
+
+struct Attribute {
+    String name;
+    String value;
+};
+
+}
+
+namespace IPC {
+
+template<>
+ErrorOr<void> encode(Encoder&, WebView::Attribute const&);
+
+template<>
+ErrorOr<WebView::Attribute> decode(Decoder&);
+
+}