Update Windows code to ffigen v16

[Levi-Lesches]

Closes #2471 by manually bumping, re-running ffigen, then cleaning up the resulting Dart code.

The big change in ffigen here is that it better handles enums, by generating actual Dart enums instead of integers

[Levi-Lesches]

Well there you have it:

Because flutter_local_notifications_windows depends on ffigen >=15.0.0 which requires SDK version >=3.4.0 <4.0.0, version solving failed.

So to merge this, we'd have to bump the SDK to ^3.4.0 and Flutter ^3.22.0, which are the next versions of Dart and Flutter. It's not a terrible change, but this PR doesn't add any new functionality so it's also not a high priority.

Feel free to mark as a draft if you want to wait on this