spin method
Spin the hue a given amount, from -360 to 360. Calling with 0, 360, or -360 will do nothing (since it sets the hue back to what it was before).
Implementation
HslColor spin(double amount) {
final hue = (h + amount) % 360;
return copyWith(h: hue < 0 ? 360 + hue : hue);
}