ENH: Add option to convert code to code-cell directives in md output

[mmcky]

This PR adds the option to convert code in rst to code-cell in markdown for use with jupyter-book and enables the direct conversion to executable code-cell directives that are available in myst.

This is enabled using:

--code-to-code-cell

in the cli or

code_to_code_cell: true

in the config.yaml

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[codecov]

Codecov Report

Merging #24 (06c8a75) into main (bb74674) will decrease coverage by 0.04%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
- Coverage   84.33%   84.28%   -0.05%     
==========================================
  Files          10       10              
  Lines        1621     1629       +8     
==========================================
+ Hits         1367     1373       +6     
- Misses        254      256       +2     

Flag	Coverage Δ
pytests	84.28% <75.00%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
rst_to_myst/mdformat_render.py	97.24% <ø> (ø)
rst_to_myst/markdownit.py	91.41% <66.66%> (-0.39%)	⬇️
rst_to_myst/cli.py	84.02% <100.00%> (+0.16%)	⬆️

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[mmcky]

@chrisjsewell I can't find the documentation on token_types -- so I don't see how to specify a myst directive when adding this token.

[mmcky]

this needs to be:

```{code-cell}
<code>
```