The styles contributed by `jupyterlab-myst` slow down JupyterLab

[krassowski]

Description

As highlighted in jupyterlab/jupyterlab#18076, jupyterlab-myst contributes hundreds of complex and slow selectors which forces browsers to attempt matching thousands of elements each time any DOM modification happens.

Proposed solution

1. Do not attach styles globally
2. Use Shadow DOM for rendering markdown cells
3. Drop the .myst prefix while in shadow DOM since not needed
4. Reuse a single stylesheet between shadow DOMs (Markdown cells)

Additional notes

The panel/bokeh ecosystem is a good example of shadow DOM usage.

[agoose77]

@krassowski thanks, yes -- this is not best practice on our part. We are working on a new release, let me make sure that we're tracking this. Thanks for dropping by!