LibWeb: Start implementing basic JavaScript DOM bindings

This patch introduces the Wrapper and Wrappable classes.

Node now inherits from Wrappable, and can be wrapped in a GC-allocated
Bindings::NodeWrapper object. The only property we expose right now is
the very simple nodeName property.

When a Document's JS::Interpreter is first instantiated, we add a
"document" property with a DocumentWrapper object to the global object.

This is pretty cool! :^)

Libraries/LibWeb/Bindings/NodeWrapper.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <LibWeb/Bindings/Wrapper.h>
+
+namespace Web {
+namespace Bindings {
+
+class NodeWrapper : public Wrapper {
+public:
+    explicit NodeWrapper(Node&);
+    virtual ~NodeWrapper() override;
+
+    Node& node() { return *m_node; }
+    const Node& node() const { return *m_node; }
+
+private:
+    virtual const char* class_name() const override { return "Node"; }
+
+    NonnullRefPtr<Node> m_node;
+};
+
+}
+}