Use new `SMAppService` for macOS 13

[sindresorhus]

Apple finally answered my prayers!

We finally don't need a useless helper app 🎉

Docs: https://developer.apple.com/documentation/servicemanagement/smappservice/3945412-mainapp?changes=latest_minor

We need to check the old property too: https://developer.apple.com/documentation/servicemanagement/smappservice/4024717-statusforlegacyplist?changes=latest_minor

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We need some migration step. My plan is to add a LaunchAtLogin.migrate() method. It would check if the old launch at login is enabled, if so, it would set the modern one to be enabled too, and then disable the old one.

Use UserDefaults to ensure the migration is only called once.