Allow setting JVM options of MillServerMain

[ken1ma]

1. We are getting StackOverflowError when compiling a case class with 35 fields and we want to increase the stack size of scalac, but there seems to be no way to do that other than setting JAVA_TOOL_OPTIONS.

   1. We are using shapeless/http4s/doobie/circe and they might be contributing to the stack consumption.

2. It would be better if we don't need to resort back to JAVA_TOOL_OPTIONS that prints a message like Picked up JAVA_TOOL_OPTIONS: -Xss4m every time a JVM is launched.

3. They say it's considered normal in Scala that you may have to increase JVM stack size in order to compile some kinds of especially involved or nested Scala code.

[ken1ma]

We have encountered the same symptom with pureconfig in a different project.
The project has only 12 case classes with 5 or less members for pureconfig to handle.