Re-export unscripted's Option, Flag, Positional, and Rest?

#286 will allow optionally annotating task parameters with them. So it would be convenient to re-export those from `package:grinder/grinder.dart`, otherwise the user will need to add a dependency on `unscripted` to their pubspec (with a version that is compatible with grinder's unscripted dependency), and then import `package:unscripted/unscripted.dart` into their `grind/tool.dart` script.

However, we would probably [need to set and maintain a very narrow constraint](https://codereview.chromium.org/775413002/) on `unscripted`. Which means that anyone who wanted to use unscripted for other scripts in their package would have a limited selection of versions, and potentially a difficult time of determining what that selection even is.