Add `minimumActiveSkeletons` setting

[Zax Ftw]

With autoAdjustMaxSkeletons enabled and low FPS, the resulting number of active skeletons is always 1 (the player). It would be nice to have SMP always active on player and follower, which is usually closest skeleton to player.

Add minimumActiveSkeletons which will clamp the resulting number of active skeletons between minimum and maximum settings.

[DaymareOn]

Excellent idea