feat(pdfx): add Swift Package Manager support

[vinnyt]

Summary

• adds Swift Package Manager support for pdfx on iOS and macOS
• consolidates the duplicated Apple implementation into shared Darwin sources
• splits the Objective-C Pigeon bridge from the Swift target as required by SwiftPM
• retains and validates CocoaPods compatibility
• updates Pigeon generation paths for the shared source layout

Validation

Validated with Flutter 3.44.9:

• flutter test: all 7 tests pass
• iOS simulator debug build using SwiftPM
• macOS debug build using SwiftPM
• iOS simulator debug build with SwiftPM disabled, using CocoaPods
• macOS debug build with SwiftPM disabled, using CocoaPods