Add support to Swift Package Manager

[ziadsarour]

Add support to Swift Package Manager as recommended by Flutter : https://docs.flutter.dev/packages-and-plugins/swift-package-manager/for-plugin-authors

[asgeirenersen]

Any plans for implementing this?
This package is now the lone holdout that prevents us from making a complete switch to Swift Package Manager.

[franboladoruiz]

Same for us. It'd be nice if Mixpanel put a bit more love into their Flutter SDK, it was the same story with WASM support.