Add portotype for Expr[Mirror] methods

[nicolasstucki]

Getting the info from the mirror in a macro is a bit tricky and verbose. The methods in this portotype intend to make this process trivial.

private def myMacroExpr[T: Type](mirror: Expr[Mirror.Of[T]])(using Quotes): Expr[Any] =
  import MirroredExpr.* // TODO where we could define this?
  val mirroredLabel: Option[String] = mirror.mirroredLabel
  val mirroredElemLabels: Option[List[String]] = mirror.mirroredElemLabels
  val mirroredMonoType: Option[Type[?]] = mirror.mirroredMonoType
  val mirroredType: Option[Type[?]] = mirror.mirroredType
  val mirroredElemTypes: Option[List[Type[?]]] = mirror.mirroredElemTypes
  ...

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[skip mima]