New Icon: Red Skull for players dead from Hardcore trait/mode (#4099)

# About the pull request

Adds a red skull to differentiate between HC death and normal perma.

# Explain why it's good for the game

Medhud and OOC observers can tell very quickly when they watch someone
die if they were HC or not. This adds a bit of cool factor to things,
and removes ambiguity for those who have that info immediately on hand
anyway.


# Testing Photographs and Procedure
Verified both with hardcore trait and gamemode enabled. Xenos also
should still only see the white skull and will not get extra info.
<details>
<summary>Screenshots & Videos</summary>

Put screenshots and videos here with an empty line between the
screenshots and the `<details>` tags.

</details>


# Changelog

:cl:
ui: Hardcore deaths are now marked with a unique red skull
imageadd: Red skull added for hardcore deaths
/:cl:

code/datums/mob_hud.dm

@@ -441,9 +441,9 @@ var/list/datum/mob_hud/huds = list(
 						holder2_set = 1
 					return
 
-				holder.icon_state = "huddead"
+				holder.icon_state = HAS_TRAIT(src, TRAIT_HARDCORE) || MODE_HAS_TOGGLEABLE_FLAG(MODE_HARDCORE_PERMA) ? "hudhcdead" : "huddead"
 				if(!holder2_set)
-					holder2.icon_state = "huddead"
+					holder2.icon_state = holder.icon_state
 					holder3.icon_state = "huddead"
 					holder2_set = 1