[DNM] Grues

Created by: Shadowmech88

Grues deal 15 brute damage per hit, can't be stunned, and move at twice the speed of humans, but they take double damage from both brute and burn. Grues also have inverted vision, meaning they see light as dark, and dark as light. This means that they can see perfectly in a pitch-black room, but can't see anything in a well-lit room.

Grues have a spell they can use to create a negative light source centered on themselves, swallowing all light around them for up to a minute. Grues can heal themselves by butchering humans with their claws and consuming human meat, 50 of each damage type is healed when they eat it.

I figure down the line they could make an interesting antag or random event or something.

This PR also adds a new charge type for spells that allows their charge to be drained gradually while the spell is active, and stop draining when the spell is deactivated, which happens when the spell runs out of charge, or can be done at any time by clicking the spell icon again. It also makes humans butcherable. They still can't be gibbed via butchering, but you can get meat and teeth from them now. I'm not sure why that was disabled.