Perserve the lexer when converting from md:myst to notebook

[mwouts]

@chrisjsewell @choldgraf I am looking for a way to preserve the cell lexer when converting the notebook from MyST Markdown to a Jupyter notebook. The reasons for that are the following:

1. I would like to preserve the lexer on round trips and solve Preserve the pygments_lexer of (unpaired) md:myst notebooks #789
2. I would like to distinguish between cells in the kernel language (that can be converted to code cells) and those in other languages (which might need a language magic), see e.g. jupytext --to md:myst *.md should be idempotent #1317

Do you have recommendations on how to best do this? Thanks

[github-actions]

Thank you for making this pull request.

Did you know? You can try it on Binder: or .

Also, the version of Jupytext developed in this PR can be installed with pip:

HATCH_BUILD_HOOKS_ENABLE=true pip install git+https://github.com/mwouts/jupytext.git@issue_789_preserve_myst_lexers

(this requires nodejs, see more at Developing Jupytext)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 91.51%. Comparing base (9aba222) to head (f2ff837).

Files with missing lines	Patch %	Lines
src/jupytext/myst.py	0.00%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1327      +/-   ##
==========================================
- Coverage   97.04%   91.51%   -5.53%     
==========================================
  Files          29       29              
  Lines        4499     4502       +3     
==========================================
- Hits         4366     4120     -246     
- Misses        133      382     +249     

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[choldgraf]

Cc @agoose77 and @rowanc1 who might have thoughts here

[agoose77]

I will think about this more broadly, but my initial reaction to #789 (comment) is that fenced-code blocks are distinctly different to code-cells, and we should not be converting them to code-cells in the MyST target format. Let me circle back here to touch on the wider topic.