Likely needs to use the low level APIs from https://github.com/flutter/packages/blob/main/packages/flutter_markdown/lib/src/builder.dart, but I think it should be possible. Alternatively we could reverse it and integrate our rich object rendering as custom elements in the markdown rendering. The second option is probably easier if it is possible, but makes us a lot more dependent on the flutter_markdown package which I don't like.

Unfortunately https://pub.dev/packages/flutter_markdown doesn't give use the low level control we need and also misses features like flutter/flutter#81739.
So we have to implement something custom, but can work with https://pub.dev/packages/markdown and copy a lot of code from https://pub.dev/packages/flutter_markdown.
We can probably also skip implementing some parts, since Talk web also only supports a limited set of features.

All supported features are tested in https://github.com/nextcloud-libraries/nextcloud-vue/blob/master/cypress/component/richtext.cy.ts, so we can implement the same subset of features (and test them as well).