First cut at unifying notebook renderers apis

This implements the api described in #123540. Major points:

- Instead of having the `markdown-it` renderer pull it its dependencies, instead the dependencies can call `getRenderer` to import the object returned by the `markdown-it` renderer

- We try to detect if a renderer is using the old or new api. Old renderers are still run as globals while new ones are loaded with `import`

- I have only hooked up the new API for markdown renderers so far