Use ES6 classes for custom OOUI widgets

[MatmaRex]

Just for a lark.

Do not use ES6 static properties though, as they're only introduced in ES2020, and also apparently everyone hates them.

[edg2s]

We should work out the limitations of this. I did this a while ago with a personal project (https://github.com/edg2s/deal/blob/master/static/model/GameModel.js), so for example OO.mixinClass is still required.

[MatmaRex]

The only limitation I noticed was that you can't set properties on this before calling super. It was trivial to avoid here, but there are some classes in OOUI and probably in Wikimedia projects where it would be more difficult. Everything else either has a direct equivalent in ES6 classes, or you can just do it by calling the OOUI methods as before.