Notebook Extend Markdown Renderer for MyST

A hacked together extension based on combining the notebook extension sample:
https://github.com/microsoft/vscode-extension-samples/tree/main/notebook-extend-markdown-renderer-sample

And the MyST-VS-code renderer extension
https://github.com/executablebooks/myst-vs-code

This is only intended as a demonstration and a hacky stopgap until notebook rendering is supported in the official myst-vs-code extension.

Was inspired in response to https://github.com/executablebooks/myst-vs-code/issues/31

This extension keeps the original :emoji: rendering support from the example, plus adds most of the MyST syntax from the original MyST extension. It utilizes the katex renderer as well to support mhchem commands.