Easier installation of oldnewthing-link-unfucker

My partner pointed out installing the oldnewthing-link-unfucker to be a bit annoyingly manual. I had not really thought about it, but that is true.

They looked into it, and it appears Greasemonkey will prompt for installation if the filename being served ends with ".user.js". I have applied that change, so if you use Greasemonkey, you should now be able to install oldnewthing-link-unfucker by navigating to the raw file on Forgejo.