feat: add macOS native plugin support via CocoaPods

[AesaKamar]

This PR introduces CocoaPods support for Monarch macOS builds, enabling the use of Flutter plugins that require native dependencies (like Rive, Firebase, etc.).

Summary of changes:

• Added 'pod install' execution to 'build_platform.dart' during the macOS build process.
• Updated 'xcodebuild' to use the '.xcworkspace' generated by CocoaPods.
• Added a generic 'Podfile' template in 'platform/macos/Podfile'.
• Updated '.gitignore' to exclude CocoaPods artifacts.

[fertrig]

Hi @AesaKamar, good to hear from you!

Are you trying to preview stories that use Rive widgets in Monarch?

This PR hardcodes Rive imports and plugin registrations in WindowManager.swift which would affect all monarch users, even the ones that don't use Rive.

I think you want a way to extend the Monarch host app with your project's native plugins. For example:

A command like monarch build macos that rebuilds the Monarch macOS host app with your project's native plugins:

1. Reads plugins from your project's pubspec.yaml
2. Generates a Podfile with those plugins
3. Runs pod install and xcodebuild
4. Outputs a custom Monarch macOS app to .monarch/ in the user's project

Users without native plugins continue using the pre-built binaries. Users with native plugins run monarch build macos once (and again when plugins change).