Add html handling to markdown cells

[fwkoch]

This should address jupyter-book/jupyterlab-myst#64 and #34 - html is now parsed as best as possible and transformed into myst.

Myst:

Before:

After:

[github-actions]

👈 Launch a Binder on branch executablebooks/jupyterlab-mystjs/feat/html

[rowanc1]

Looks good -- works well. This is parsing the html, which is different than the final solution we will likely want (e.g. people are expecting HTML to actually be rendered verbatim).

We may want to have some sort of option to turn this on eventually.