Support exercise and solution node in myst-to-md

[kp992]

This PR tries to add exercise and solution node in md converter. The error looks like:

⛔️ exports/a.ipynb Unsupported node type: exercise
⛔️ exports/a.ipynb Unsupported node type: solution

cc @agoose77 @mmcky

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Latest commit: d9e208d

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[kp992]

The diff fixes the error but the output is still empty. So I believe I might need to fix directiveHandlers, can you please review @agoose77?

Thank you.

[kp992]

@agoose77 I have updated the PR with some tests. Can you please review and share your suggestions and improvements?

[rowanc1]

PR looks good in general! I am trying to think of a way to not have the extension as a dependency?

[kp992]

Thanks for the review @rowanc1. Hmm, do you have any suggestions for that?