Add threading to top level pull request comments

[fitz-vivodyne]

See #552 which was closed as outdated.

Not being able to have threaded discussions on pull requests is very annoying.

There's something of a workaround in being able to comment/start a thread on unrelated code, but it would be a much better user experience if it just worked.

[ankneis]

Hi @fitz-vivodyne! Thank you for your participation - we will be closing this specific Discussion. Here are a number of feedback discussions that cover this topic—feel free to upvote and share your feedback where it fits best, or create a new discussion if needed!

• https://github.com/orgs/community/discussions/8469
• https://github.com/orgs/community/discussions/5633