Add Swift Package Manager support for iOS

[wytzepiet]

Summary

Flutter is deprecating CocoaPods-based plugin integration and will require Swift Package Manager (SPM) in a future version. Currently, fjs triggers the following warning at build time:

The following plugins do not support Swift Package Manager for ios:
  - fjs
This will become an error in a future version of Flutter.

Context

• Flutter's SPM migration guide: https://docs.flutter.dev/packages-and-plugins/swift-package-manager/for-plugin-authors
• This is currently a warning but will become a build error in a future Flutter release

Environment

• fjs v2.1.0
• Flutter 3.x (stable)
• iOS deployment

Thanks for the great package!