Planes & Layers part 1: The Re-Layering Redux (#18741)

This is part 1 of a project to, eventually, bring the render planes
system from /tg/ and bay into Aurora. This is a prerequisite and blocker
for many things the development team and community want to have, and
this was long overdue.

Many objects have been re-assigned layers, which are now thoroughly
defined. Maps have had their custom layer defines purged, as we should
be moving away from this in favor of saner definitions in the base
items.

This should be a test-merge due to the sheer amount of layers changed,
which will very likely create issues that I cannot possibly discover and
debug in a reasonable amount of time myself.

---------

Co-authored-by: Cody Brittain <cbrittain10@live.com>

code/modules/random_map/automata/diona.dm

@@ -12,7 +12,7 @@
 	anchored = TRUE
 	density = TRUE
 	opacity = FALSE
-	layer = TURF_LAYER + 0.01
+	layer = ABOVE_TILE_LAYER
 	var/max_health = 50
 	var/health
 	var/destroy_spawntype = /mob/living/carbon/alien/diona