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/game/objects/items/debug_items.dm

@@ -140,6 +140,7 @@
 			to_chat(user, span_reallybig("You shouldn't have done that..."))
 			playsound(src, 'sound/voice/borg_deathsound.ogg')
 			sleep(3 SECONDS)
+			living_user.investigate_log("has been gibbed by [src].", INVESTIGATE_DEATHS)
 			living_user.gib()
 			return
 	var/turf/loc_turf = get_turf(src)