LibWeb: Add JSON serialization method to DOM::Node

This method builds a JSON object representing the full state of the
DOM tree.

The JSON that is built will be used for building the DOM Inspector
widget for the OutOfProcessWebView.

Userland/Libraries/LibWeb/DOM/Node.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/Badge.h>
+#include <AK/JsonObjectSerializer.h>
 #include <AK/RefPtr.h>
 #include <AK/String.h>
 #include <AK/TypeCasts.h>
@@ -162,6 +163,9 @@ public:
 
     bool is_host_including_inclusive_ancestor_of(const Node&) const;
 
+    // Used for dumping the DOM Tree
+    void serialize_tree_as_json(JsonObjectSerializer<StringBuilder>&) const;
+
 protected:
     Node(Document&, NodeType);