allow mainargs to work with scala-cli without the --objectWrapper flag

[benwbooth]

I would like to be able to use mainargs with scala-cli. Right now using mainargs with the @main annotation on a case class causes a compiler error due to the way scala-cli wraps the program code by default. A workaround is to run scala-cli with the --objectWrapper flag, which changes the way scala-cli wraps the program code to make it compatible, but doing so causes scala-cli to emit a warning message about the --objectWrapper flag being an unstable experimental feature. lt would be nice if the @main annotation could be reworked to support running in scala-cli without requiring the use of the --objectWrapper flag.