Mutant Entities

Additions:

- Added mutant skeletons.

- Mutant skeleton ability: can do melee attacks and shoot arrows

- The arrows mutant skeletons shoot do more damage than normal.

- A mutant skeleton can spam arrows while a partially mutant skeleton shoots them once every few seconds.

Changes to existing features:

- Changed mutant husks ability a bit, instead of placing sand blocks it spawns a larger area of sand and teleports above the sand.

- Changed nuclear wastelands generation a bit, slightly higher base height and a bit less height variation.