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/game/objects/structures/crates_lockers/closets/secure/security.dm

@@ -21,7 +21,7 @@
 
 /obj/structure/closet/secure_closet/captains/populate_contents_immediate()
 	new /obj/item/gun/energy/e_gun(src)
-	new /obj/item/storage/belt/sabre(src)
+	new /obj/item/storage/belt/sheath/sabre(src)
 
 /obj/structure/closet/secure_closet/hop
 	name = "head of personnel's locker"

code/game/objects/structures/mystery_box.dm

@@ -55,7 +55,7 @@ GLOBAL_LIST_INIT(mystery_box_extended, list(
 	/obj/item/clothing/gloves/rapid,
 	/obj/item/shield/riot/flash,
 	/obj/item/grenade/stingbang/mega,
-	/obj/item/storage/belt/sabre,
+	/obj/item/storage/belt/sheath/sabre,
 	/obj/item/knife/combat,
 	/obj/item/melee/baton/security/loaded,
 	/obj/item/reagent_containers/hypospray/combat,