Bastion Turret

New turrets (Large Turret Base):
- Railgun: the anti-boss gun. Each shot takes a Rail Slug and 8,000 FE. The coils light up one by one,
  electricity crawls over the barrel, then a slug flies at 40 blocks per tick: 60 damage that ignores armor,
  double against bosses, and a shockwave that throws mobs around the impact away. Always aims at the
  biggest target in range.
- Mortar: lobs shells on a high arc over walls at mobs on the ground (never flyers). It shells the spot where
  its target stands, so it's best against crowds; a lone mob that keeps walking gets away. Each shell leaves a
  3x3 fire patch for 5 seconds that burns everyone standing in it, you and your animals too, and sets
  burnable blocks on fire with real fire (server config "mortarBlockFire" turns that off).
- New ammo: Rail Slugs and Mortar Shells (Ammo Workstation). New parts for both turrets.

Balance:
- Tesla Coil: lightning now hits up to 5 targets per discharge (was 2), at most 2 bolts per target.
  A lone mob still takes 2 bolts (20 damage). FE cost unchanged (3,000 per discharge).
- Shotgun: 5 damage per pellet (was 2.5). Point blank, one blast kills a zombie.