LibJS: Support o.f++ :^)

This patch teaches UpdateExpression how to use a Reference. Some other
changes were necessary to keep tests working:
A Reference can now also refer to a local or global variable. This is
not fully aligned with the spec since we don't have a Record concept.

Libraries/LibJS/Interpreter.cpp

@@ -173,7 +173,7 @@ Reference Interpreter::get_reference(const FlyString& name)
                 return { Reference::LocalVariable, name };
         }
     }
-    return { &global_object(), PropertyName(name) };
+    return { Reference::GlobalVariable, name };
 }
 
 void Interpreter::gather_roots(Badge<Heap>, HashTable<Cell*>& roots)