Layer coffins above beds (#4399)

# About the pull request
Makes coffins layer above beds

# Explain why it's good for the game

![image](https://github.com/cmss13-devs/cmss13/assets/41448081/1262a3a6-7836-4a25-80d4-91324438f017)

To take Soldier out of context,

![image](https://github.com/cmss13-devs/cmss13/assets/41448081/bcce725d-58e8-4583-a604-1356894211f2)

This is a borderline exploit that can only be found by right clicking on
every single tile that has a bed, and is better off dead.

# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>


![image](https://github.com/cmss13-devs/cmss13/assets/41448081/cfec1eb6-89b7-42df-801f-96d096af606e)

</details>


# Changelog
:cl:
balance: Coffins now layer above beds
/:cl:

code/__DEFINES/layers.dm

@@ -83,10 +83,15 @@
 
 #define ABOVE_SPECIAL_RESIN_STRUCTURE_LAYER 3.01
 
+/// A layer above objects (like structures) but below items
+#define BETWEEN_OBJECT_ITEM_LAYER 3.01
+
+/// The layer on which items lay
+#define ITEM_LAYER 3.02
 /// for items that should be at the top of the pile of items
-#define UPPER_ITEM_LAYER 3.01
+#define UPPER_ITEM_LAYER 3.03
 /// just above all items
-#define ABOVE_OBJ_LAYER 3.02
+#define ABOVE_OBJ_LAYER 3.04
 
 #define BUSH_LAYER 3.05
 

code/game/objects/items.dm

@@ -2,8 +2,8 @@
 	name = "item"
 	icon = 'icons/obj/items/items.dmi'
 	mouse_drag_pointer = MOUSE_ACTIVE_POINTER
+	layer = ITEM_LAYER
 	light_system = MOVABLE_LIGHT
-
 	/// this saves our blood splatter overlay, which will be processed not to go over the edges of the sprite
 	var/image/blood_overlay = null
 	var/randpixel = 6

code/game/objects/structures/crates_lockers/closets/coffin.dm

@@ -6,6 +6,7 @@
 	icon_opened = "coffin_open"
 	material = MATERIAL_WOOD
 	anchored = FALSE
+	layer = BETWEEN_OBJECT_ITEM_LAYER
 
 /obj/structure/closet/coffin/update_icon()
 	if(!opened)