add support for _repr_markdown_ IPython methods

[cscheid]

This addresses an issue with quarto users who expect the behavior of Python cells to more closely match those of quarto's Jupyter engine.

I'm happy to add supporting tests/documentation, though I could use some help navigating your expectations on that!

(@kevinushey thanks for the help!)

[t-kalinowski]

Hi @cscheid this looks great. Thank you very much!

As far as tests and docs I think this should be sufficient:

• A news entry
• An additional chunk or chunks added to this Rmd, or, if you'd prefer, a stand-alone Rmd similar to the "eng-reticulate-example.Rmd" that gets rendered as part of a test like this.
• If there is a particular python package you want us to test against please add to the GHA setup here

Thanks again!

[kevinushey]

LGTM!