Add a mapping helper for welding airlocks (tgstation#74206)

## About The Pull Request
What it says on the tin. Warning: copy-pasted mapper sprites.

## Why It's Good For The Game

Fewer varedits, better maintainability, more obvious in the map editor
what's been applied to a given airlock.


## Changelog

No player facing changes.

_maps/RandomRuins/SpaceRuins/oldstation.dmm

@@ -8402,9 +8402,9 @@
 /area/ruin/space/ancientstation/beta/atmos)
 "XD" = (
 /obj/machinery/door/airlock/maintenance_hatch{
-	name = "Custodial Closet";
-	welded = 1
+	name = "Custodial Closet"
 	},
+/obj/effect/mapping_helpers/airlock/welded,
 /obj/effect/decal/cleanable/dirt,
 /obj/machinery/duct,
 /turf/open/floor/iron,

code/modules/mapping/mapping_helpers.dm

@@ -216,6 +216,15 @@
 	else
 		airlock.abandoned = TRUE
 
+/obj/effect/mapping_helpers/airlock/welded
+	name = "airlock welded helper"
+	icon_state = "airlock_welded"
+
+/obj/effect/mapping_helpers/airlock/welded/payload(obj/machinery/door/airlock/airlock)
+	if(airlock.welded)
+		log_mapping("[src] at [AREACOORD(src)] tried to make [airlock] welded but it's already welded closed!")
+	airlock.welded = TRUE
+
 /obj/effect/mapping_helpers/airlock/cutaiwire
 	name = "airlock cut ai wire helper"
 	icon_state = "airlock_cutaiwire"

tools/UpdatePaths/Scripts/74206_airlock_weld_help.txt

@@ -0,0 +1 @@
+/obj/machinery/door/airlock/@SUBTYPES{welded = 1} : @OLD{@OLD;welded=@SKIP}, /obj/effect/mapping_helpers/airlock/welded