LibJS: Make AssignmentExpression assign through a Reference

Reference now has assign(Interpreter&, Value) which is used to write
transparently through a Reference into whatever location it refers to.

Libraries/LibJS/Runtime/Reference.h

@@ -71,6 +71,8 @@ public:
         return m_local_variable;
     }
 
+    void assign(Interpreter&, Value);
+
 private:
     Value m_base { js_undefined() };
     PropertyName m_name;