Add compat for higherKinds compiler flag

[nrktkt]

It would be handy to be able to import scala.language.compat.higherKinds and have that not generate a deprecation warning when building on 2.13.

This could be as simple as

package scala.language

object compat {
  implicit lazy val higherKinds = scala.languageFeature.higherKinds
}