Add Swift Package Manager support for iOS and macOS

[muhammedeminalan]

Summary

• add Package.swift for iOS plugin target
• add Package.swift for macOS plugin target
• add SwiftPM source layouts under ios/flutter_tts/Sources/flutter_tts and macos/flutter_tts/Sources/flutter_tts
• add a small iOS Swift wrapper class (FlutterTtsPlugin) so plugin registration remains compatible with pluginClass: FlutterTtsPlugin
• keep CocoaPods files untouched for backward compatibility

Validation

• flutter analyze passes in plugin root
• swift package dump-package passes for ios/flutter_tts
• swift package dump-package passes for macos/flutter_tts

Fixes #633

[muhammedeminalan]

Hi maintainer, this PR adds Swift Package Manager support for iOS and macOS to address #633.

What’s included:

• Package.swift for iOS and macOS plugin folders
• SwiftPM source layout
• iOS wrapper class to keep pluginClass compatibility
• CocoaPods flow remains unchanged

Validation:

• flutter analyze passes
• swift package dump-package passes for both ios/flutter_tts and macos/flutter_tts

Happy to make any requested changes quickly. Thanks!

[woxxin20]

@dlutton can plan to release new version with support SMP