Feat: Add Desktop Support (Windows & macOS)

[rkishan516]

Currently, the mixpanel-flutter SDK provides excellent support for mobile platforms (iOS and Android) and web. However, we should also add support for desktop platforms (Windows, macOS, and Linux).

Developers building Flutter applications that target desktop platforms (specifically Windows and macOS, though Linux would also be welcome) in addition to mobile cannot currently use mixpanel-flutter for their desktop builds. This forces them to:

1. Implement custom/separate analytics solutions for desktop.
2. Use conditional logic to exclude Mixpanel on desktop builds.
3. Look at alternatives which support flutter desktop

We propose extending the mixpanel-flutter SDK to include official support for Windows and macOS desktop platforms. This would likely involve:

1. Creating the necessary native implementations (e.g., using C++, Swift/Objective-C for macOS, C++/WinRT for Windows) or leveraging any existing native Mixpanel SDKs for these platforms.
2. Integrating these native implementations into the existing Flutter plugin architecture.
3. Ensuring the Flutter API remains consistent (as much as possible) with the mobile implementation, allowing developers to use the same Mixpanel integration code across all platforms.

Flutter is increasingly becoming a extremely viable choice (including for us at EFG) and it would be a shame to not support it directly.

[JonasJW]

We need desktop support, please!