chore: add pull request template picker and userstyle update template

[uncenter]

I've seen @isabelroses comment about updating the version when someone fixes a userstyle a million times and I think it might help if we have a PR template checklist for that. Turns out GitHub doesn't have a pull request template picker (like the issue template picker) so you have to do this little workaround with query strings.

[isabelroses]

I like this PR and I have no reasons to reject it.
But I would like to hear other people opinions.

[uncenter]

Totally. I realize this is a different kind of change so I'll let you guys (maintainers) figure out if this is a good thing to implement going forward.

[uncenter]

Here's a demonstration of it in action:

Screenshot.2023-11-14.at.11.32.10.Arc.-converted.mp4

It works by appending a query string linking to a template, see https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/using-query-parameters-to-create-a-pull-request.

You can fork my test repo, make a change, and try it out yourself: https://github.com/uncenter/pull-request-template-test.