Fixes wooden coffins deconstructing into metal sheets

Sure, I /could/ fix the messages and the fact that a wooden coffin is
deconstructed with a welder, but just think of it as slicing the hinges
apart because I'm too lazy.
Also makes sarcophaguses deconstruct into sandstone, but I don't think
I've ever seen those in-game.
🆑
fix: Coffins will now deconstruct into wood instead of metal.
/🆑

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

@@ -6,6 +6,7 @@
 	icon_opened = "coffin_open"
 	burn_state = FLAMMABLE
 	burntime = 20
+	material_drop = /obj/item/stack/sheet/wood
 
 /obj/structure/closet/coffin/update_icon()
 	if(!opened)
@@ -18,4 +19,5 @@
 	icon_state = "sarc"
 	icon_closed = "sarc"
 	icon_opened = "sarc_open"
-	sound = 'sound/effects/stonedoor_openclose.ogg'
+	sound = 'sound/effects/stonedoor_openclose.ogg'
+	material_drop = /obj/item/stack/sheet/mineral/sandstone