A {@link CompositeMoveSelector} that unions 2 or more {@link MoveSelector}s.
For example: a union of {A, B, C} and {X, Y} will result in {A, B, C, X, Y}.
Warning: there is no duplicated {@link Move} check, so union of {A, B, C} and {B, D} will result in {A, B, C, B, D}.
@see CompositeMoveSelector