Updates `local_auth_windows` to use Pigeon, and moves to a more platform-tailored and Dart-centric implementation (rather than keeping the previous cross-platform method channel interface that the current implementation was duplicated from):
- Eliminates `deviceSupportsBiometrics` from the platform interface, since it's always the same as `isDeviceSupported`, in favor of doing that mapping in Dart.
- Eliminates `getEnrolledBiometrics` from the platform interface, since it was the same implementation as `isDeviceSupported` just with a different return value, in favor of doing that logic in Dart.
- Moves throwing for the `biometricOnly` option to the Dart side, simplifying the native logic.

Related changes:
- Adds a significant amount of previously-missing Dart unit test coverage.
- Removes the `biometricOnly` UI from the example app, since it will always fail.

Part of https://github.com/flutter/flutter/issues/117912