[desktop_drop] support for Swift Package Manager

[ekuleshov]

Plugin needs to support Swift Package Manager on iOS and MacOS platforms.

See Flutter 3.41 release notes at
https://blog.flutter.dev/whats-new-in-flutter-3-41-302ec140e632#97e2:~:text=Swift%20Package%20Manager%20and%20UIScene

Also see the plugin developer guide at
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

Expected behavior

Swift Package Manager supported on iOS and MacOS platforms

Version (please complete the following information):

• Flutter Version: 3.41
• OS: ios, macos
• plugin: desktop_drop: 0.7.0