improve default help page for new users #5977
Description
When a user types flutter with no arguments, show common actions first with more detailed information so that the new user can get started quickly...
Common actions:
flutter create <output directory>
Create a new Flutter project in the specified directory.
flutter analyze [options] [<target>]
Analyze Flutter project's Dart code.
flutter run [options]
Run your Flutter application on an attached device
or in an emulator.
... existing help output here ...