Add Qt QML/JS engine to MDN (Browser compatibility data)

[ivan tkachenko]

Hi,

MDN Web Docs (formerly known as the Mozilla Developer Network or MDN) is a
great resource for learning about JavaScript features and their availability
is various web browsers and other runtimes.

I want Qt's own QML/JS engine (technically known as QV4) to be 
represented in
the browser compatibility table.

In Qt 5.15 the engine lacks several major features. In Qt 6.x situation has
got a lot better, but still not on parity with latest standards: for example
we can use Nullish coalescing operator (??) already but not Nullish
coalescing assignment (??=) just yet. It would make it easier for developers
to know which API is available in case they have to work on older/LTS
versions.

I intend to start a discussion and see which steps need to be taken to 
get us
there. Feel free to leave your two cents and give it a thumbs up at GitHub!

https://github.com/orgs/mdn/discussions/401

-- ivan (@ratijas)