Add possibility to sort export from barrel files

[rodrigobastosv]

Describe the solution you'd like
It would be great if we could have the possibility to sort the export from our barrel files

Additional context
Let's supose we have a barrel file like this:

export 'a.dart';
export 'c.dart';
export 'd.dart';
export 'b.dart';
export 'e.dart';

It would be great if the tool also sorts these kind of files. The expected result would be something like this:

export 'a.dart';
export 'b.dart';
export 'c.dart';
export 'd.dart';
export 'e.dart';

[github-actions]

👋 Hello! Thanks for submitting a issue! @Matt-Gleich will try to respond as soon as possible.

[gleich]

See #63