1.7.1: Add cross-process notification support for App Groups

- Add limitToInstance parameter to @ObservableDefaults macro (default: true)
- Enable system-wide UserDefaults notification listening when set to false
- Essential for App Group scenarios where widgets/extensions modify shared UserDefaults
- Maintain backward compatibility with default behavior unchanged

Key improvements:
- Support cross-process synchronization between main app and extensions
- Fix issue where @Default macro cannot trigger view refresh with App Groups enabled
- Add comprehensive documentation for App Group usage
- Include tests for cross-process notification behavior

Note: When using limitToInstance: false, always use a unique prefix to filter
notifications from the intended suiteName and avoid conflicts with other
UserDefaults instances.