Fixes #106701

This PR adds a new extension point that allows extensions to either replace or augment the rendering of markdown in notebooks. To do this, we've moved the rendered markdown from our main editor dom into the back layer webview of the notebook.

Todos

• Make a more thorough test of accessibility with these changes
• When pre-rendering the notebook cells on first load, look at cell position cache instead of just getting first five cells
• Move the math contribution into a new extension
• Make sure that only core extensions can fully replace the markdown renderer for now
• Figure out markdown extensions should be contributed. Right now they come in the same way replacement markdown renderers do
• Improve drag + drop of cells. Currently there are noticeable delays