I am aware there is a Joplin Safari Web Clipper here from cweirup called JoplinSafariWebClipper.
Which is a native web clipper for Safari and excellent work!
However, Apple released a Extension Convertor and I thought I would try it on the vanilla Chrome Extension:
It works perfectly after running the 1 command I posted there, without changes and I couldn't find any bugs or things not working.
So I believe this would be the way to add this Extension (when the certificates are added) into the mainline and the App Store for Safari Extensions.