Support Swift Package Manager

[alexobviously]

Flutter now finally supports SPM, and cocoapods is getting deprecated pretty soon, so it would be a good idea to update this to support SPM. I don't think it's a big change - I looked at doing it myself and making a PR but the structure of your project is a little bit different to the other packages I'm using for reference and I'm not sure how I'd test it, so I thought it's better to leave it to the person who actually understands the package better.

https://docs.flutter.dev/packages-and-plugins/swift-package-manager/for-plugin-authors#how-to-add-swift-package-manager-support-to-an-existing-flutter-plugin

This is the dynamsoft dependency: https://github.com/Dynamsoft/barcode-reader-spm

Happy to help if needed!