Sable Sub-level Loading System Fix

Changes how Sable determines when and how sublevels are loaded and unloaded.

Prevents sub-levels from unloading when the only player loading them is dead, fixes Sable's sub-level loading and unloading when a player is on a sub-level but its holding chunk is unloaded, and keeps sub-levels loaded when any chunk within the player-requested view distance contains any part of that sub-level (even if no player is standing on it).

Useful for modpacks/instances in which you intend to create very large sub-levels that span many chunks.

Requires Sable 2.0.3 or newer.