Rich text editor can generate potentially invalid commonmark

Reported on Meta.SE

Describe the bug

Overlapping bold/italic formatting leads to incorrect formatting when toggling to Markdown mode.

1. In rich text mode, write some text.
   • this is bold, now italic, and now?!
2. Select part of the text.
3. Make the selection italic.
   • this is bold, now italic, and now?!
4. Select some overlapping text.
5. Make the selection bold.
   • this is bold, now italic, and now?!

(Now bold and italic formatting is correctly applied to overlapping portions of the text.)

When switching to Markdown, the formatting is screwed up because Markdown doesn’t support overlapping formatting in this way: adjacent */** is ambiguous at the best of times, but Markdown wouldn’t support this even when using _ to format italics.

Here’s what the editor generates:

**this is bold, *now italic****, and now?!*

… but as mentioned, even the following isn’t really supported by Markdown as far as I’m aware (it certainly isn’t on this site):

**this is bold, _now italic**, and now?!_