Silverfish now give a little Slayer XP on Survival

Key changes

• Silverfish you kill yourself on Survival now give a small amount of Slayer XP.
• The amount is deliberately tiny, one of the smallest amounts the game pays and on a par with a sheep, pig or chicken, so a silverfish farm cannot out-earn a zombie or spider farm.
• Only the player who lands the killing blow is credited, so kills from spawners or other automation earn nothing.