[FEATURE] Add comment to pull request

[hardcover-pessimist]

I'd like to be able to add comments to a pull request.

upstream supports it.

[craftamap]

Hey - sorry for not responding on this.

This is something I want to support if I find the time for it. I guess it's a bit difficult to get the design for it right though, since we probably want to support leaving comments at a specific position in a file as well as responding to comments.