[mixin_logger] support for Swift Package Manager

[lockieRichter]

mixin_logger needs to support Swift Package Manager on iOS.

With Swift Package Manager enabled, Flutter reports:

The following plugins do not support Swift Package Manager for ios:

• mixin_logger

The plugin's ios/ directory currently ships only a CocoaPods podspec (mixin_logger.podspec) and has no Package.swift, so it can't be resolved via SPM and SPM-enabled projects fall back to CocoaPods for it.

See the Flutter plugin-author guide for adding SPM support:
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

(Related: #466 requests the same for desktop_drop.)

Expected behavior

Swift Package Manager supported on iOS — a Package.swift added alongside the existing podspec — so the warning goes away and SPM-enabled projects no longer need the CocoaPods fallback for this plugin.

Version (please complete the following information):

• Flutter Version: 3.44.5
• OS: ios
• plugin: mixin_logger: 0.1.3