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/Wrapper.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <AK/Weakable.h>
+#include <LibJS/Object.h>
+#include <LibWeb/Forward.h>
+
+namespace Web {
+namespace Bindings {
+
+class Wrapper
+    : public JS::Object
+    , public Weakable<Wrapper> {
+protected:
+    explicit Wrapper() {}
+};
+
+}
+}