Add iOS SPM support and CI verification

[kevmo314]

Summary

• add Swift Package Manager support for the iOS plugin using a pure Swift package layout
• keep CocoaPods support in place for compatibility and update publish validation accordingly
• add macOS CI coverage that verifies the example app with both CocoaPods and SPM

Verification

• flutter analyze
• swift package dump-package
• GitHub Actions will verify iOS builds for both package managers on this PR

Closes #12