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/instruments/items.dm

@@ -30,9 +30,9 @@
 	if(user.incapacitated() || !((loc == user) || (isturf(loc) && Adjacent(user)))) // sorry, no more TK playing.
 		return STOP_PLAYING
 
-/obj/item/instrument/suicide_act(mob/user)
+/obj/item/instrument/suicide_act(mob/living/user)
 	user.visible_message(span_suicide("[user] begins to play 'Gloomy Sunday'! It looks like [user.p_theyre()] trying to commit suicide!"))
-	return (BRUTELOSS)
+	return BRUTELOSS
 
 /obj/item/instrument/attack_self(mob/user)
 	if(!ISADVANCEDTOOLUSER(user))