Add investigate_deaths (#71112)

## About The Pull Request
Adds INVESTIGATE_DEATHS, an investigate category intended to better show
causes of death.


![image](https://user-images.githubusercontent.com/66640614/200142461-c17b5e51-1116-4eef-bbfb-49bc024c0953.png)


![image](https://user-images.githubusercontent.com/66640614/200147306-09bef76e-68c6-4f0a-bdf9-0211eb274e66.png)

Also makes suicide_act take a `mob/living` as an argument instead of a
`mob`, and some minor style improvements since apparently I hate
atomicity.

## Why It's Good For The Game
Inspired by a mysterious death and dusting. More logging and leads for
admins investigating deaths.

Also fixes #59028

## Changelog
🆑 Tattle
admin: added investigate deaths to shed some more light on unusual
demises, dustings, and gibbings
/🆑

Co-authored-by: tattle <article.disaster@gmail.com>

code/modules/mob/living/basic/basic_defense.dm

@@ -153,6 +153,7 @@
 			if(prob(bomb_armor))
 				adjustBruteLoss(500)
 			else
+				investigate_log("has been gibbed by an explosion.", INVESTIGATE_DEATHS)
 				gib()
 				return
 		if (EXPLODE_HEAVY)