feat(ios): add Swift Package Manager support

[nurlangarash]

Summary

Adds a Package.swift manifest alongside the existing podspec so superwallkit_flutter builds with Flutter 3.44+ Swift Package Manager (SPM) while remaining fully CocoaPods-compatible.

Under Flutter 3.44+ the plugin currently emits:

The following plugins do not support Swift Package Manager for ios: … superwallkit_flutter … This will become an error in a future version of Flutter.

This PR removes that warning. CocoaPods is not removed — both dependency managers consume the same source tree during the transition (CocoaPods becomes read-only on 2 Dec 2026).

Changes

• Move ios/Classes/ → ios/superwallkit_flutter/Sources/superwallkit_flutter/ (shared source tree used by both SPM and CocoaPods). All moves are git mv renames, so history is preserved.
• Add ios/superwallkit_flutter/Package.swift:
  • swift-tools-version: 5.9, platform iOS 14.0 (matches the podspec floor).
  • Depends on the native SuperwallKit SPM package (github.com/superwall/Superwall-iOS), pinned exact: "4.14.2" to match s.dependency 'SuperwallKit', '4.14.2'.
  • Follows the official Flutter-team plugin convention (dependencies: [] for Flutter itself — the engine is injected automatically by the SPM build; no Flutter package is declared).
• Update ios/superwallkit_flutter.podspec source_files to the new shared path.
• Ignore SPM build artifacts (.build/, .swiftpm/, Package.resolved).

The plugin is 100% Swift and uses Pigeon, so no Objective-C umbrella/header handling is needed — the Pigeon-generated SuperwallHostGenerated.swift simply moves with the other sources.

Reference

Followed the official Flutter guide for plugin authors: https://docs.flutter.dev/packages-and-plugins/swift-package-manager/for-plugin-authors, cross-checked against the SPM manifests in flutter/packages (e.g. url_launcher_ios, shared_preferences_foundation).

Testing status

⚠️ Please run CI / a device build before merging. I authored the source restructure and manifest by following the official conventions and verified the SuperwallKit SPM product name/version resolve, but I was not able to run a full iOS device/simulator build in my environment. Suggested verification:

flutter config --enable-swift-package-manager
cd example && flutter clean && flutter run        # SPM path
flutter config --no-enable-swift-package-manager
cd example && flutter clean && flutter run        # CocoaPods path (backward-compat)

Happy to adjust the platform floor, version pin (e.g. .upToNextMajor(from: "4.0.0")), or layout to match your preferences.