Unable to be launched directly as a command.

After installing with

```dart
dart pub global activate import_sorter
```
the import_sorter command cannot be used directly.

It’s recommended to support this feature for easier reuse of import_sorter across multiple projects.

For example, I’m using it with Melos, where a project contains multiple apps/packages, and I plan to use a single command to organize all the code. Currently, I can only use:

```
dart pub global run import_sorter:main
```
which is quite awkward.