Upgrade to CodeMirror 5.59.2

[cben]

Closes #97. Hopefully helps #81. May unblock some math improvements (mode multiplexing has been improved).

CodeMirror git repo no longer contains codemirror.js, requires build. => Using my fork with commits containing codemirror.js, created using https://github.com/cben/CodeMirror/blob/build-releases/build-releases.sh generating tags such as https://github.com/cben/CodeMirror/tree/5.40.0-build/

• [ ] default line spacing is increased, is that desirable?

[cben]

https://deploy-preview-180--elastic-liskov-92f696.netlify.app/?doc=issue97

• [ ] Headers are now small!