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/DOM/Document.h

@@ -51,6 +51,8 @@ class StyleSheet;
 
 class Document : public ParentNode {
 public:
+    using WrapperType = Bindings::DocumentWrapper;
+
     Document();
     virtual ~Document() override;