Update ECMAScript version

ECMAScript 2021 has been finalized while QJSEngine is still only implementing ES2016 features.
A lot of these features make the language easier to use in async contexts (most notably async/await async iteration and additions to the Promise API). It would be great to be able to use these in Qt. ES2021 features like such private methods would also make it easier to design easy to use QML components.

It would be great if we could write more modern JS in Qt if it supported a newer ECMAScript standard.