Add `teeingCollector` to `MiscellaneousApiProvider`

[tlinkowski]

  /**
   * Equivalent of {@link Collectors#teeing(Collector, Collector, BiFunction)}.
   */
  <T, R1, R2, R> Collector<T, ?, R> teeingCollector(Collector<? super T, ?, R1> downstream1,
          Collector<? super T, ?, R2> downstream2, BiFunction<? super R1, ? super R2, R> merger);

More JDK 12 APIs:

• https://dzone.com/articles/39-new-features-and-apis-in-jdk-12