Splits placeontop proc (#79702)

## About The Pull Request
I find the proc hard to read honestly. There's no reason we can't split
this into two functions - the secondary functionality is used only once,
in reader.dmm.
## Why It's Good For The Game
Code improvement
Glorious snake case
## Changelog
N/A nothing player facing

---------

Co-authored-by: san7890 <34697715+san7890@users.noreply.github.com>

code/modules/mapping/mapping_helpers.dm

@@ -153,11 +153,11 @@
 			if(1 to 9)
 				var/turf/here = get_turf(src)
 				for(var/turf/closed/T in range(2, src))
-					here.PlaceOnTop(T.type)
+					here.place_on_top(T.type)
 					qdel(airlock)
 					qdel(src)
 					return
-				here.PlaceOnTop(/turf/closed/wall)
+				here.place_on_top(/turf/closed/wall)
 				qdel(airlock)
 				qdel(src)
 				return