I was able to solve the problem by doing a clean install of macOS and Xcode. What a dumb as hell problem. No idea what the cause was. I did try removing Xcode and relevant files/directories first, but that didn't help. Only the clean install of macOS seemed to do the trick.

I still have the same question as @hmpro74. My extension is not visible to Safari and I have been unable to find any troubleshooting steps.

This looks like a similar or perhaps identical problem I'm experiencing https://developer.apple.com/forums/thread/668693