Suggest --scalac-help prominently

[lrytz]

scala-cli 1.1.3

Neither scala-cli -help nor scala-cli compile -help suggest a way to get -help from the compiler itself, i.e., scala-cli -S <version> --scalac-help (or -O -help might be more educative).

I think this is quite important for scala-cli to become the default runner for Scala releases, it should be shown prominently in both cases.

Older (closed) ticket: #1308.