Add an optional prefix for Recurse

[corey-at-protenus]

Only implemented Scala 3. Once confirmed this is the appropriate direction I'll implement for Scala 2.

Looks like:

final case class FewArgs(
  value: String = "default",
  numFoo: Int = -10
)

final case class RecurseWithPrefix(
  noPrefix: Int,
  @Recurse("prefix") withPrefix: FewArgs
)

Will result in the options --no-prefix, --prefix-value and --prefix-num-foo.

Had a hard time figuring out where to modify the name formatter to do this. I thought I'd be able to update the Recurse parsers default name formatter but no luck.
See #86

[alexarchambault]

@corey-at-protenus This looks neat! Feel free to keep working on that…

[corey-at-protenus]

Should be ok to review.

I'm passing around an empty Recurse instead of an Option[Recurse]. Eh. Might change that.

[corey-at-protenus]

Howdy! Any further changes required before merge?