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/DocumentWrapper.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <LibWeb/Bindings/NodeWrapper.h>
+
+namespace Web {
+namespace Bindings {
+
+class DocumentWrapper : public NodeWrapper {
+public:
+    explicit DocumentWrapper(Document&);
+    virtual ~DocumentWrapper() override;
+
+    Document& node();
+    const Document& node() const;
+
+private:
+    virtual const char* class_name() const override { return "Document"; }
+};
+
+}
+}