Add Monterey (macOS 12.0+) support

Fixes https://github.com/aiaf/Stillcolor/issues/7 To add macOS 12.0+ support, I had to: - Downgrade `LaunchAtLogin` support for macOS 12 following the official guide [here](https://github.com/sindresorhus/LaunchAtLogin-Legacy). This step also increases app size due to the package bundling. - Replace `MenuBarExtra` as it is only supported since macOS 13 This is my first time working with Swift, so I had some challenges identifying the predecessor to `MenuBarExtra`. Even though my current approach is compatible with macOS 12, there may be more effective methods for creating menu bar applications on macOS 12. However, for macOS 13+ I definitely recommend using your implementation.