Support default values in case classes in Scala 3

[lolgab]

Currently, default values in case classes are not handled correctly in Scala 3.
Example:

object Main {
  @mainargs.main
  case class Config(bar: String = "bar")

  def main(args: Array[String]): Unit = {
    val config = mainargs.ParserForClass[Config].constructOrExit(args)
    println(config)
  }
}

Scala 2 prints:

Missing argument: --bar <str>
Expected Signature: apply
  --bar <str>

Scala 2 prints:

Config(bar)