Polish notebook markdown extension point

[mjbvz]

Follow up on #106701, which added a new extension point that lets extensions contribute markdown extensions for notebooks.

To enable the new render API, set:

"notebook.experimental.useMarkdownRenderer": true,

Todos

• Make a more thorough test of accessibility with these changes
  • Improve focus tracking
  • Test screen reader
• 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

[mjbvz]

I've split the remaining issues out:

#121256 For API review
#118594 for accessibility