LibJS: Implement the object literal __proto__ property key special case

Author: https://github.com/IdanHo
Commit: 7ebb421ee9
Pull-request: https://github.com/SerenityOS/serenity/pull/12897
Issue: https://github.com/SerenityOS/serenity/issues/12896
Reviewed-by: https://github.com/davidot ✅
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibJS/AST.h

@@ -1648,6 +1648,7 @@ public:
         Getter,
         Setter,
         Spread,
+        ProtoSetter,
     };
 
     ObjectProperty(SourceRange source_range, NonnullRefPtr<Expression> key, RefPtr<Expression> value, Type property_type, bool is_method)