Migrate plugin to Swift Package Manager for iOS

[elasticdog]

I'm using flutter_midi_command in my Flutter app, and with the latest upstream release of Flutter 3.44.0, there is now a deprecation warning upon compilation about the lack of support for the Swift Package Manager. I'd like to request adopting the new structure, as it will become an error in a future version of Flutter.

I'd be glad to lend a hand with this process, and there's also some upstream documentation regarding the migration:

• How to add Swift Package Manager support to an existing Flutter plugin