iOS: Add Swift Package Manager support and update project structure
- Added Swift Package Manager manifest for iOS integration
- Moved Objective-C sources to new layout under ios/video_thumbnail/Sources/
- Updated podspec to reflect new source file locations
- Raised minimum iOS deployment target to 11.0 for SPM compatibility
- Added README documentation for SPM integration
- Updated CHANGELOG with new features
Co-Authored-By: Claude noreply@anthropic.com