Update to CodeMirror 6

[hildjj]

The online page currently uses an old version of CodeMirror 5. We would like to upgrade to CodeMirror 6.

This would be a great first bug if you've worked with CM6 before, and would like to contribute to Peggy. It's likely that the pegjs language mode for CM5 will also need to be rewritten for this to work. In that case, we'll be happy to host a new project in the PeggyJS GitHub org, and give you control over that repo.

[reverofevil]

1. Monaco editor isn't really that hard to set up. Why not switch to an editor that can support something like this? (ugliest code alert; press Next and hover in either column);
   
2. Would you consider a PR with more serious rewrite for the /online page? I probably know where the idea of CodeMirror came from, but probably could do better this time.
   

[hildjj]

Yes, I'd look at a PR that switched to something that isn't CodeMirror. I'm not tied to CM at all, and maintenance costs matter to me, so simpler is better particularly for non-core functionality.

[Mingun]

Yeah, I was actually also thinking, would not switch to monaco?

Having dark theme also would be very appreciating!

[hildjj]

I am strongly in favor of moving to Monaco, after having tried it on another project.