feat: Add Swift Package Manager support and restructure iOS implementation
- Add Swift Package Manager support for iOS implementation
- Move iOS sources into new ios/uri_to_file layout and update podspec
- Align iOS method channel name with Dart API
- Fix Swift Package Manager builds by removing Objective-C bridge layer
- Update version to 1.2.0
- Update README with Swift Package Manager installation instructions
Co-Authored-By: Claude noreply@anthropic.com