[FR] Make Markdown support more robust

[MikeWallaceDev]

1~3 main use cases of the proposed feature I want to be able to export more elaborate structure to Markdown I want to be sure that Markdown export will function and have sensible defaults when it doesn't recognize a tag

what types of users can benefit from using your proposed feature Any user who wishes to export a Document to Markdown

Additional context We have an internal structure called Delta, we must ensure that every tag in Delta can be exported to Markdown. Markdown itself does not support all of the features that Delta does, but this can be handled with HTML tags. However, it is (apparently) not recommended to use HTML tags. So we shouldn't just go adding them willy-nilly.