slideY method
Adds a SlideEffect that moves the target vertically based on a fraction of its size
per the specified begin and end values (via SlideTransition).
Implementation
T slideY({
  Duration? delay,
  Duration? duration,
  Curve? curve,
  double? begin,
  double? end,
}) {
  begin ??= end == null ? SlideEffect.defaultSlide : SlideEffect.neutralSlide;
  end ??= SlideEffect.neutralSlide;
  return addEffect(SlideEffect(
    delay: delay,
    duration: duration,
    curve: curve,
    begin: SlideEffect.neutralValue.copyWith(dy: begin),
    end: SlideEffect.neutralValue.copyWith(dy: end),
  ));
}