De-snowflake thrown objects hitting turfs.

* Port of https://github.com/Baystation12/Baystation12/pull/16942
* Actually tell turfs when a thrown object hits them, and let them decide what to do about it!
  * We do this by calling hitby(), which is how it already works for obj and mob, so this makes behavior consistent.
* This allows us to cleanly solve the problem of a thrown object landing on open space without falling.

code/modules/multiz/turf.dm

@@ -36,6 +36,11 @@
 	..()
 	mover.fall()
 
+// Called when thrown object lands on this turf.
+/turf/simulated/open/hitby(var/atom/movable/AM, var/speed)
+	. = ..()
+	AM.fall()
+
 /turf/simulated/open/proc/update()
 	below = GetBelow(src)
 	turf_changed_event.register(below, src, /turf/simulated/open/update_icon)