[local_auth] Add possibility to use error_codes to catch specific exceptions 

In version 1.1.11 it was possible to import error_codes as : import 'package:local_auth/local_auth.dart/error_codes.dart' as auth_error;

In version 2.0.0 as package started using strict exports there is no way to access error codes.
Consider adding :

```
export ‘package:local_auth/src/types/error_codes.dart’ as ErrorCodes 

```
to packages/local_auth/local_auth/lib/local_auth.dart