add Selective

https://www.staff.ncl.ac.uk/andrey.mokhov/selective-functors.pdf

trait Selective[F[_]]  {
  def applicative: Applicative[F] // we could also extend, but that may create ambiguities
  // the law here is if you give `F[Right[B]` we never evaluate `fn`
  def select[A, B](fab: F[Either[A, B]])(fn: F[A => B]): F[B]
}

object Selective {
  def fromMonad[F[_]: Monad]: Selective[F] =
    new Selective[F] {
      def applicative = Monad[F]
      def select[A, B](fa: F[Either[A, B]])(fn: F[A => B]): F[B] =
        fa.flatMap {
          case Right(b) => Monad[F].pure(b)
          case Left(a) => fn.map(_(a))
        }
    }
}

Note the paper gives some interesting examples of things that don't have Monad but do have Selective, e.g. Validated.

Start by reading the linked selective functors paper and the existing Applicative and Monad abstractions in the Cats repository. Clarify the API and laws, including the proposed fromMonad behavior and support for Validated, then identify the tests needed to establish the design before implementation.