Refactors firestacks into status effects (#66573)

This PR refactors firestacks into two status effects: fire_stacks, which behave like normal firestacks you have right now, and wet_stacks, which are your negative fire stacks right now. This allows for custom fires with custom behaviors and icons to be made.

Some fire related is moved away from species(what the fuck was it even doing there) into these as well.
Oh and I fixed the bug where monkeys on fire had a human fire overlay, why wasn't this fixed already, it's like ancient.

Also changed some related proc names to be snake_case like everything should be.

This allows for custom fire types with custom behaviours, like freezing freon fire or radioactive tritium fire. Removing vars from living and moving them to status effects for modularity is also good.
Nothing to argue about since there's nothing player-facing

code/modules/awaymissions/mission_code/snowdin.dm

@@ -6,7 +6,7 @@
 	requires_power = FALSE
 	static_lighting = FALSE
 	base_lighting_alpha = 255
-	
+
 /area/awaymission/snowdin/outside
 	name = "Snowdin Tundra Plains"
 	icon_state = "awaycontent25"
@@ -219,7 +219,7 @@
 		burn_human.visible_message(span_warning("[burn_human]'s [burn_limb.name] melts down to the bone!"), \
 			span_userdanger("You scream out in pain as your [burn_limb.name] melts down to the bone, leaving an eerie plasma-like glow where flesh used to be!"))
 	if(!plasma_parts.len && !robo_parts.len) //a person with no potential organic limbs left AND no robotic limbs, time to turn them into a plasmaman
-		burn_human.IgniteMob()
+		burn_human.ignite_mob()
 		burn_human.set_species(/datum/species/plasmaman)
 		burn_human.visible_message(span_warning("[burn_human] bursts into a brilliant purple flame as [burn_human.p_their()] entire body is that of a skeleton!"), \
 			span_userdanger("Your senses numb as all of your remaining flesh is turned into a purple slurry, sloshing off your body and leaving only your bones to show in a vibrant purple!"))