[WIP]: Upgrade Codemirror to v6

[lindapaiste]

Fixes #2081 (when complete)

I haven't looked at this in a while but I figured I might as well put in a draft.

Primary Goal:

• Upgrade to v6

Secondary Goals:

• Don't modify/overwrite core Codemirror files. Use plugins to extend and modify instead.
• Do more of the HTML rendering in React instead of vanilla JS (searchbox, etc.).
• Separate code for various functionalities (linting, formatting, etc.).

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[raclim]

Hey! I wanted to check in on this and ask if this PR was something you'd still be interested in working on!

After speaking with some folks recently, I noticed that a lot of them showed strong interest in seeing Collaborative Editing happen. Although I'm not sure if this particular feature is something we're committed to implementing yet, I think getting this upgrade down would be a great and needed first step regardless!

I was also wondering since you were working on this if this was something you also feel would be feasible using @codemirror/collab?