Markdown outputs are not rendering in ipywidgets

[stevejpurves]

Describe the bug

context
When I do use any of the examples that have markdown cell output
https://executablebooks.github.io/thebe/ipywidgets.html

expectation
I expected markdown to be rendered

bug
But instead I get plain text happens

problem
This is a problem for people!

Reproduce the bug

run this and look at the output of the cookies example https://executablebooks.github.io/thebe/ipywidgets.html

List your environment

No response

[stevejpurves]

The issue stems from changes to the IRendermimeRegistry interface in jupyterlab.

Now this requires a separate markdown renderer, even to render markdown mimetype outputs and seems to be now inextricably linked with codemirror editor languages, even though it's a concern of output rendering for mimebundles.

We'll need to supply a markdown renderer of some form to the rendermime registry, using the marked renderer as-per jupyterlab will mean pulling in @juptyerlab/codemirror or building out a stub implementation of IEditorLanguageRegistry :( . Probably it's better to consider a different/better parser and use that? maybe with the need to wrap it, potentially we can just use mystjs here cc @rowanc1?