It practically already exists. Xcode can import an existing WebExtension and generate an Xcode project for it. Assuming you are happy with the default behaviour

It practically already exists. Xcode can import an existing WebExtension and generate an Xcode project for it. Assuming you are happy with the default behaviour of the companion app (which basically just gives you info on the extension, whether it is enabled in Safari, and how to enable it if not), you can just hit build and submit it to Apple.