fix(Comments.ts)!: Add support for threaded comments (#1194)

* chore(Comments.ts): Improve error messages a bit

* feat(CommentThread.ts): Add support for sub threads

* fix(CommentThread.ts): Handle subthread being empty in `is_prepopulated`
> Happens when the old format is received.
> This doesn't fix the problem that this isn't backward compatible, though.

* chore(tests): set `generate_session_locally` to false
> Let's see if this works inside Github Actions