People become desensitized to death from exposure (#94924)

## About The Pull Request

Desensitized is no longer binary yes/no, now scales from 0.1x to 1x (or
beyond, I guess)

Desensitized jobs start as 0.5x desensitized (which is the threshold for
being considered "truly desensitized")
Some antags are now 0.1x to 0.25x desensitized

Witnessing death of a fellow human gives you slight desensitization
(currently -0.025x).
A normal crewmember, after witnessing 20 deaths, is on par with a
roundstart desensitized crewmember.
Likewise a desensitized crewmember has almost no reaction to death after
witnessing 20 deaths.
Your own deaths count towards this value.

There's an achievement for managing to go from 1x to 0.1x across the
course of an entire round.

## Why It's Good For The Game

This is intended to contribute to the "story" people face across the
length of a round.
Rounds with few overall deaths results in crewmembers regularly getting
shocked, but bloodbaths results in crewmembers "dehumanizing and facing
the bloodshed".

## Changelog

🆑 Melbert
add: Desensitization to death is no longer binary - some antagonists are
now more used to it than others.
add: Witnessing the death of a fellow humanoid (or dying yourself) will
slightly desensitize you to future deaths.
add: Adds an achievement for managing to max out desensitization across
a round.
add: Desensitized crewmembers care less when splattered with blood.
add: Holodeck mobs have a reduced death mood impact.
/🆑

code/modules/holodeck/holo_effect.dm

@@ -63,7 +63,7 @@
 	our_mob.flags_1 |= HOLOGRAM_1
 
 	// these vars are not really standardized but all would theoretically create stuff on death
-	our_mob.add_traits(list(TRAIT_PERMANENTLY_MORTAL, TRAIT_NO_BLOOD_OVERLAY, TRAIT_NOBLOOD, TRAIT_NOHUNGER), INNATE_TRAIT)
+	our_mob.add_traits(list(TRAIT_PERMANENTLY_MORTAL, TRAIT_NO_BLOOD_OVERLAY, TRAIT_NOBLOOD, TRAIT_NOHUNGER, TRAIT_SPAWNED_MOB), INNATE_TRAIT)
 	RegisterSignal(our_mob, COMSIG_QDELETING, PROC_REF(handle_mob_delete))
 	return our_mob