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/mapfluff/ruins/spaceruin_code/hilbertshotel.dm

@@ -381,6 +381,7 @@ GLOBAL_VAR_INIT(hhMysteryRoomNumber, rand(1, 999999))
 	// Let's gib the last person to have selected a room number in it.
 	if(unforeseen_consequences)
 		to_chat(unforeseen_consequences, span_warning("\The [H] starts to resonate. Forcing it to enter itself induces a bluespace paradox, violently tearing your body apart."))
+		unforeseen_consequences.investigate_log("has been gibbed by using [H] while inside of it.", INVESTIGATE_DEATHS)
 		unforeseen_consequences.gib()
 
 	var/turf/targetturf = find_safe_turf()