fix!(ipynb writer): improve round trip identity

for raw cell output

BREAKING CHANGE:
The Jupyter ecosystem, including nbconvert, lab and notebook,
deviated from their own spec in nbformat,
where they used the key `raw_mimetype` instead of `format`.

Moreover, the mime-type of rst used in Jupyter
deviated from that suggested by
https://docutils.sourceforge.io/FAQ.html
and is defined as `text/restructuredtext`
when chosen from "Raw NBConvert Format" in Jupyter.

So while this is backward-compatible,
it should matches the real world usage better,
hence improving the round-trip "identity" in raw-cell.

See #229, jupyter/nbformat#229.

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