Figure out how to resolve Markdown dependency in OSS and google repository

[jacobsimionato]

Currently, we use markdown-it, but this isn't available in the Google repo.

Some possible ideas:

• Somehow allow the google repo to use a different dependency than the Github repo
• Modularize the system so that google internal apps can share the core low-dependency code, but fork other parts of it. For example, if we clearly split the renderer framework (e.g. A2UImessageProcessor) etc from the catalog implementation, then in the Google repo we can fork the Catalog implementation and use our internal markdown renderer, while Github can use its one. This also has the advantage that Google teams can rapidly iterate on the specific visual styling for each element without modifying open source code.

[ditman]

I'd claim this issue for myself but don't have permissions in the repo!

If you want to merge a fix before I do, don't feel bad (but cc me in the PR, please!)