Make cats.syntax.flatMap include untilDefinedM

[travisbrown]

untilDefinedM is a method on FlatMap and should be available via cats.syntax.flatMap, but it's not:

scala> import cats.instances.list._, cats.syntax.flatMap._
import cats.instances.list._
import cats.syntax.flatMap._

scala> List(Option(1)).untilDefinedM
<console>:18: error: value untilDefinedM is not a member of List[Option[Int]]
       List(Option(1)).untilDefinedM
                       ^

It is in all:

scala> import cats.instances.list._, cats.syntax.all._
import cats.instances.list._
import cats.syntax.all._

scala> List(Option(1)).untilDefinedM
res0: List[Int] = List(1)

This change just fixes this oversight. Like #3305 it's not currently tested, but will be in #3304.

[kailuowang]

Thanks

[codecov-io]

Codecov Report

Merging #3306 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3306   +/-   ##
=======================================
  Coverage   93.15%   93.15%           
=======================================
  Files         378      378           
  Lines        7585     7585           
  Branches      214      214           
=======================================
  Hits         7066     7066           
  Misses        519      519

Flag	Coverage Δ
#scala_version_212	93.4% <ø> (ø)	⬆️
#scala_version_213	92.93% <ø> (ø)	⬆️

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