Add support for Markdown files

[bveeramani]

This PR adds support for MyST syntax like:

```{testcode} group
:skipif: True

print('ham')
```  

```{testoutput}
:options: +NORMALIZE_WHITESPACE

ham
```

[bveeramani]

Hey @Thisch, could you please take a look? If the high-level implementation looks good, I'll clean it up and add tests.

[twmr]

Hey @bveeramani! Thx a lot for your contribution. Sry that it took me so long to review your change. Are you still interested in adding tests and cleaning up the change? If not, I'm going to merge this change and add test in a later commit.

[bveeramani]

No worries! Unfortunately, I don't think I'll have time to clean up and add tests for this change :(