feat: migrate iOS/macOS to Swift Package Manager

[AlexV525]

Summary

Migrate image_editor_common iOS and macOS native code to Swift Package Manager, following the same SPM layout as flutter_image_compress.

Changes

Source restructuring (iOS + macOS)

• Classes/ → <platform>/image_editor_common/Sources/image_editor_common/*.m (flat)
• Headers → Sources/image_editor_common/include/image_editor_common/*.h
• Preserved as git mv (R100) — full history retained

Swift Package Manager

• Added Package.swift for iOS (.iOS("9.0")) and macOS (.macOS("10.15"))
• Product name image-editor-common, target image_editor_common
• cSettings: [.headerSearchPath("include/image_editor_common")] for local header resolution
• No pubspec swiftPackageManager key — Flutter auto-discovers Package.swift

Podspec compatibility

• Both podspecs updated to reference the new Sources/ paths
• CocoaPods builds remain fully functional (backward compatible)

Example & tooling

• Example iOS project migrated for Flutter SPM integration
• .gitignore: added **/.build/ and Flutter/ephemeral/

Verification

flutter build ios --debug --no-codesign
→ ✓ Built build/ios/iphoneos/Runner.app
→ Adding Swift Package Manager integration... (3.8s)
→ FlutterGeneratedPluginSwiftPackage resolves image_editor_common via SPM
→ image_editor_common NOT in Podfile.lock (confirmed via SPM)

Backward compatibility

Both CocoaPods and SPM paths work against the same source tree. The CI matrix here exercises the CocoaPods path; SPM can be verified locally with flutter config --enable-swift-package-manager.