I'm using Safari for personal use these days. I wanted to use omnibear for micropub stuff, so I compiled the JS (a normal build step for it), and then ran the command below. The only change I had to make was to add a description to the manifest.
xcrun safari-web-extension-converter /path/to/my/extension/
In Safari, the Allow Unsigned Extensions option must be checked. Xcode generates a wrapper application for omnibear. After booting it, you can quit it and go to Safari -> Preferences -> Extensions to enable the extension. I'm posting this note using it!