Write RawBlock of markdown in code-cell output.
#7561 makes the ipynb reader reads code-cell output with mime
"text/markdown" to a RawBlock of markdown
This commit makes the ipynb writer writes this RawBlock of markdown
back inside a code-cell output with the same mime, preserving this
information in round-trip
Add tests of ipynb reader (#7561) and ipynb writer (#7563)'s ability to
handle a "text/markdown" mime type in a code-cell output
