generic parent type for both sabre sheaths (#92300)

## About The Pull Request
puts both the grass and normal sabre sheath types under a shared parent
type for shared behavior.
considered subtyping the storage datum but it seemed overkill. can do if
requested.
## Why It's Good For The Game
less copy paste. good if someone wants to add a clay-more sheath for
similar (believe that exists somewhere downstream), would have done it
myself but no sprites.
## Changelog
N/A

code/modules/deathmatch/deathmatch_loadouts.dm

@@ -585,7 +585,7 @@
 	neck = /obj/item/bedsheet/captain
 	glasses = /obj/item/clothing/glasses/sunglasses
 	gloves = /obj/item/clothing/gloves/captain
-	belt = /obj/item/storage/belt/sabre
+	belt = /obj/item/storage/belt/sheath/sabre
 	l_hand = /obj/item/gun/energy/laser/captain
 	r_pocket = /obj/item/assembly/flash
 	l_pocket = /obj/item/melee/baton/telescopic