Change how spawn & death positions are tracked by Player

[dktapps]

Problem description

Currently we need some clunky hacks to avoid returning Position containing unloaded worlds.

Proposed solution

Since we don't actually need the World instance until getSpawn() or getDeathPosition() are actually called, it would probably make more sense to store a foldername + Vector3, and then create a Position just-in-time when getSpawn() or getDeathPosition() are called. This would also permit spawn & death positions in unloaded worlds to be saved irrespective of whether the target world is loaded, allowing the world to be later reloaded without breaking runtime Positions that we didn't actually need anyway.

Related to #5502

This is (mostly) non-BC breaking. It would only affect custom Player classes, so it would probably be OK to make a change like this in minor-next.