LibWeb: Add Function To Build Accessiblity Tree

With this patch, the accessibility tree can be build from the root
node of a document. This can then be serialzed and sent to (soon
to come) consumers.

Userland/Libraries/LibWeb/DOM/AccessibilityTreeNode.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2022, Jonah Shafran <jonahshafran@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/JsonObjectSerializer.h>
+#include <AK/Vector.h>
+#include <LibJS/Heap/Cell.h>
+#include <LibWeb/DOM/Node.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::DOM {
+
+class AccessibilityTreeNode final : public JS::Cell {
+    JS_CELL(AccessibilityTreeNode, JS::Cell)
+public:
+    static JS::NonnullGCPtr<AccessibilityTreeNode> create(Document*, DOM::Node const*);
+    virtual ~AccessibilityTreeNode() override = default;
+
+    JS::GCPtr<DOM::Node> value() const { return m_value; }
+    void set_value(JS::GCPtr<DOM::Node> value) { m_value = value; }
+    Vector<AccessibilityTreeNode*> children() const { return m_children; }
+    void append_child(AccessibilityTreeNode* child) { m_children.append(child); }
+
+    void serialize_tree_as_json(JsonObjectSerializer<StringBuilder>& object) const;
+
+protected:
+    virtual void visit_edges(Visitor&) override;
+
+private:
+    explicit AccessibilityTreeNode(JS::GCPtr<DOM::Node>);
+
+    JS::GCPtr<DOM::Node> m_value;
+    Vector<AccessibilityTreeNode*> m_children;
+};
+
+}