Add reference syntax highlighting grammar

[chrisjsewell]

Myst is designed to be CommonMark compliant, therefore standard Markdown syntax highlighters will work with Myst and handle the common syntaxes. However, it would be desired to have an 'extended' syntax highlighter, to cover the additional syntax introduced by Myst.

The de facto standard for syntax grammar is the TextMate grammar file, with support in Eclipse, Atom, VSCode, Sublime, etc (jupyterlab/jupyterlab#5504 also discusses their potential incorporation). Therefore, it would be good to have a reference implementation (tested somehow?) in this package.

Here are some helpful links:

• https://www.apeth.com/nonblog/stories/textmatebundle.html
• https://benparizek.com/notebook/notes-on-how-to-create-a-language-grammar-and-custom-theme-for-a-textmate-bundle
• https://medium.com/@model_train/creating-universal-syntax-highlighters-with-iro-549501698fd2

[chrisjsewell]

It should also be considered whether the specific syntaxes discussed in executablebooks/MyST-NB#12 would be incorporated here. For example with:

```{execute} python
print('hi')
```

It would be desired that the block content was syntax highlighted as python code.

Perhaps there could be a script that auto-generated grammar files with additional highlighting rules for specific directives.

[choldgraf]

I think it sounds useful - how is this different from the LSP stuff?

Is the idea that the textmate grammar file would be the "official" reference implementation, then?

[chrisjsewell]

how is this different from the LSP stuff?

This is essentially a pre-cursor to a full LSP. LSP uses TextMate bundles for static syntax highlighting (there's actually no way yet to do it dynamically microsoft/language-server-protocol#18). The LSP then builds on this to add dynamic stuff like document outlines, auto-completion, linting, etc.

Is the idea that the textmate grammar file would be the "official" reference implementation, then?

Yep, then it could be used by editor specific extension packages, like for VS Code or Jupyter Lab (hopefully).

[chrisjsewell]

FYI @jstac @mmcky (see the readme for use)

[jstac]

Beautiful! @mtiley, @najuzilu, if you're using VS Code for the transfer project, this will significantly increase your QOL.