Swift Package Manager support

[Adrian-Samoticha]

pub.dev reports the following platform support-related issue:

0/0 points: Swift Package Manager support
Package does not support the Swift Package Manager on macOS
It does not contain macos/macos_window_utils/Package.swift.

This package for iOS or macOS does not support the Swift Package Manager. It will not receive full points in a future version of the scoring model.

See https://docs.flutter.dev/to/spm for details.

[EchoEllet]

As an update, this has been fixed in #66. See also Adrian-Samoticha/appkit_ui_element_colors#1.