Codemirror 6

[felixroos]

It would be nice to update the REPL to Codemirror 6.
Here are some examples of what it brings to the table: https://codemirror.net/6/examples/
I especially like:

• decoration widgets for custom ui elements
• collab editing
• custom gutters

I already tested react-codemirror6 and it's pretty easy to integrate.
You can see it in action as part of this post (The highlighting is still buggy when an event is triggered while already highlighted.).

[felixroos]

this should be a good starting point: https://github.com/felixroos/felixroos.github.io/blob/main/components/strudel/CodeMirror.jsx

[yaxu]

A big plus is that this will make strudel more accessible e.g. to people using screen readers. It'd be good to do some testing around that.

[felixroos]

Implemented with #102