Add iOS Swift Package Manager support

[yanncabral]

Summary

• Add iOS Swift Package Manager support for the Flutter plugin.
• Move the iOS Swift source into the Flutter SPM package layout.
• Keep CocoaPods support by updating the podspec source path and version.
• Enable SPM in the example app and validate flutter build ios --no-codesign on macOS CI.

Validation

Local validation available in this workspace:

• Parsed pubspec.yaml, example/pubspec.yaml, and .github/workflows/publish.yml as YAML.
• Parsed ios/open_settings_plus.podspec with pod ipc spec.
• Parsed the SwiftPM manifest with swift package --package-path ios/open_settings_plus dump-package using a temporary local FlutterFramework stub.
• Checked the iOS example project with xcodebuild -list -project example/ios/Runner.xcodeproj.

Full Flutter/iOS validation is expected to run in GitHub Actions because the local workspace does not have flutter/dart installed.

Related issues

Fixes #28
Fixes #40