Lint for .flatTap with non-Unit result

[armanbilge]

flatTap is defined as:

def flatTap[A, B](fa: F[A])(f: A => F[B]): F[A] =
  flatMap(fa)(a => as(f(a), a))

https://github.com/typelevel/cats/blob/fbad4be326cb1a244bfae14db43e06cc486aa602/core/src/main/scala/cats/FlatMap.scala#L172-L173

While silently discarding an arbitrary B is convenient for ergonomics, it may be desirable to call attention to the fact you are discarding a result by requiring that B =:= Unit and thus forcing the user to call .void if their function does not return F[Unit].

This seems like a good candidate for a lint. Inspired by a Discord question, h/t @sbuzzard.