LibWeb: Expose Node.appendChild() to the web

This is a very barebones implementation of appendChild() that doesn't
take any of the idiosyncratic DOM behaviors into account yet.

Also teach the wrapper generator how to turn an Interpreter argument
into a Node&.

Libraries/LibWeb/DOM/Node.idl

@@ -8,5 +8,6 @@ interface Node : EventTarget {
     readonly attribute Node? parentNode;
     readonly attribute Element? parentElement;
 
+    Node appendChild(Node node);
 }