Add Swift Package Manager support for macOS

[angie313]

Hi, we are getting the following warning message when running macOS builds, could you please add support for Swift Package Manager?

The following plugins do not support Swift Package Manager for macos:
  - local_notifier
This will become an error in a future version of Flutter. Please contact the plugin maintainers to request Swift Package Manager adoption.

[goodspb]

Hi @angie313, I've submitted a PR that adds Swift Package Manager support for macOS: #34.