Copy-pasting markdown into `compose` should probably traverse MDAST pipeline

[huumn]

re: https://stacker.news/items/1461017

If I copy-paste rich text into write, it converts it to markdown, preserving the styling. We should probably support the same behavior in compose. If I copy-paste markdown into compose, outside of a code block, it should convert it to rich text.

It's clear (to me) that stackers are not aware of the editor modes, so we have to assume they aren't and assume the most common case - which I think it the above behavior.

[Soxasora]

I absolutely agree and I'd like to see this at some point. It was one of the goals of universal copy/paste, but the heuristics needed to make this work also made it somewhat difficult to conceptualize.

Scanning for Markdown in rich payloads seems out of discussion, because we would need to find a way to parse markdown hidden inside of HTML/Lexical nodes. So we need to check plain text data for probably-markdown and convert it for the user, in the hope that we don't catastrophically ruin other copy/paste operations that contain unintentional markdown syntax.

This needs some thinking. I was also thinking about reusing Lexical's MarkdownShortcuts plugin, that can convert markdown into rich Lexical nodes while writing; probably we can exclusively use it to paste in compose mode.