Enhancement: dryrun mode

[preminger]

Describe the feature
Support an execution mode where mage only prints the command-lines it would run, but doesn't actually run them.

What problem does this feature address?
Allows you to see what would be done before it actually happens. While -v and -debug already exist, they both still result in the actual commands being executed; in some situations, one may want to verify the correctness of one's magefile by just seeing what would be running.

[jamesainslie]

Loving what mage allows - particularly in super complex orchestration environments. But this enhancement would be 🤌