Add --command and --scratch-dir to run command

[alexarchambault]

scala-cli run --command basically prints the command scala-cli run would have run (with one argument per line). This can be passed over to xargs for example, but users can also manually pre-process these arguments if they feel like it. This is also useful when debugging for example (it was for me when working on #1129).

--scratch-dir makes scala-cli run write temporary stuff (launchers, .js files, …) inside the passed directory rather than in a temporary OS location, and makes Scala CLI not delete these temporary files. When used in conjunction with --command, that allows users to later run the command, even if it involves temporary files.

(Bits from #1129, that don't involve Spark.)

[mpkocher]

Perhaps it would be useful to consider renaming this to --scratch-dir to avoid confusion with the standard env varsTMPDIR, TEMP mechanism of defining a temporary directory?

The current naming scheme (e.g., --tmp-dir) is perhaps a little bit confusing and I suspect the behavior would not be completely obvious.