Implement custom codemirror modes

[laudep]

Note: this been asked before in the legacy app: #1866.

Code snippets in languages that are not supported by CodeMirror modes by default are not highlighted.
CodeMirror maintainers have stated:

We're not taking requests for modes, or even adding new modes to the main distribution in general. You'll have to implement (and distribute) this yourself.

I think adding custom modes to Boostnote can increase the adoption rate and all-round user experience.
I for one would also like to have ABAP support for example.

[Rokt33r]

I found https://github.com/larshp/codemirror-abap. Is it stable enough? https://github.com/larshp/codemirror-abap

[laudep]

Yeah it should be.
I've recently updated that repo to upgrade the dependencies and the highlighting.