Explore customizing the markdown renderer for notebooks

[mjbvz]

Explores letting extensions customize the markdown rendering used in notebooks

[mjbvz]

Update on our current plans:

• Branch with current work

You can test out the new extensible markdown support by setting "notebook.experimental.useMarkdownRenderer": true

Fully replacing the markdown renderer

The new notebookMarkdownRenderer contribution point lets an extension contribute its own markdown render fore notebook cells. This contribution is very similar to an output renderer contribution. The contributed renderer code is run inside the backlayer webview and the rendered markdown is also shown in this webview instead of in the main editor dom like our built-in markdown support

For now, we will restrict this API to our internal extensions. We already have an extension that replaces the notebook's markdown renderer with markdown-it, which we believe is extensible enough for most common notebook uses. In the future, we may open this API to let external extensions use their own markdown engine too.

Extending the markdown renderer

Extensions can contribute extensions to the new markdown-it based markdown renderer. Example use cases include adding support for math and emoji in markdown.

These markdown extensions also come in through the notebookMarkdownRenderer contribution point. They also contribute scripts that are run inside the webview. However rather than replacing the markdown renderer, they instead call the extendMarkdownIt global function and are given an instance of markdown-it which they can extend.

For now, we plan on including a simple markdown-it math extension in VS Code. In the future, math support may instead come from an external extension, such as Python or even a dedicated math extension.

TODO

Short term

• Improve markdown cell drag and drop
• Make sure links in markdown cells are handled properly
• Investigate accessibility for markdown cells
• Make sure new markdown cells don't cause other regressions
• Use markdown-it as the default renderer for notebooks

Longer term

• Move math support out of VS Code (either to python or another extension)
• Create markdown notebook extensions for other features, such as emoji
  • Should these be part of our existing markdown-preview extensions?
• Figure out how a notebook tells us which markdown features should/shouldn't be enabled
• Remove our in-editor rendering once it is no longer needed

/cc @rebornix @kieferrm

[rchiodo]

Math support might be possible with one of these?
https://www.npmjs.com/package/markdown-it-latex
https://www.npmjs.com/package/markdown-it-texmath

[mjbvz]

Merged initial API point. Will continue working on this with #116393