Add possibility to sort export from barrel files

**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';
```