Fixes per world difficulty with /difficulty command and also
makes it so that the server keeps the last difficulty used instead
of restoring the server.properties every single load.
Expose an entities spawn reason on the entity.
Pre existing entities will return NATURAL if it was a non
persistenting Living Entity, SPAWNER for spawners,
or DEFAULT since data was not stored.
Additionally, add missing spawn reasons.
Co-authored-by: Jake Potrebic <jake.m.potrebic@gmail.com>
Co-authored-by: Doc <nachito94@msn.com>
