Add Swift Package Manager support for iOS and macOS

[Shonu72]

Summary

Adds Swift Package Manager support for the flutter_js plugin, resolving #190.

This keeps the plugin compatible with Flutter's transition to SwiftPM while preserving existing CocoaPods support.

Changes

• Added SwiftPM package manifests for iOS and macOS.
• Moved Apple platform sources into the SwiftPM Sources/flutter_js structure.
• Preserved the iOS Objective-C forwarding shim for CocoaPods compatibility.
• Updated the iOS and macOS podspecs to use the new source paths.
• Added SwiftPM build directories to .gitignore.
• Removed obsolete empty Apple platform source/resource directories.

Validation

Tested with Flutter 3.47:

• iOS build with SwiftPM enabled
• macOS build with SwiftPM enabled
• iOS build with SwiftPM disabled using CocoaPods
• macOS build with SwiftPM disabled using CocoaPods
• Both Package.swift manifests parse successfully
• Both podspecs parse successfully
• git diff --check passes