Expose "corpses persist" flag to Lua

Creatures and NPCs have a "persistent" flag that blocks their corpse from despawning, and prevents the player from disposing of their body. There's currently no way to check this from Lua, which is a blocker for de-hardcoding the Container window.

NPC and Creature records already have fields for the other flags isEssential and isRespawning , so it would make sense to add a simple isPersistent alongside them.