pub command in dart and flutter

With a lot of new Dart users coming from the flutter space, the details on how to run the tests etc. might need to be updated.

E.g., replace/give alternatives to `pub run test` with `flutter pub run test` or `dart run test` in https://exercism.io/tracks/dart/tests

From the current docs: [dart.dev/tools/pub/cmd](https://dart.dev/tools/pub/cmd)
> Flutter note: If you’re using the Flutter SDK, don’t use the pub command directly. Instead use the flutter pub command, as described in Using packages on the Flutter website.

And I think the separate pub command will be depreciated soon anyway: [dart.dev/tools/dart-tool](https://dart.dev/tools/dart-tool)
> Version note: Before Dart 2.10, the dart command was used only to run the Dart VM. You can still use it that way, but now the command has more functionality. Over time, we expect the dart command to replace most existing commands in the Dart SDK.