Refactored the abstract meta propriety (#19797)

Refactored the abstract meta propriety into defines.
It's now more easy to spot/see abstract types thanks to the macro that
defines them.
Added a check on initialization of atoms to avoid spawning abstract
types.
Made the spawn_atom proc check for abstractness.
Made the spawn_atom proc use tgui_list for types list shorter than 1000
elements, which enables to search in them. It's too laggy on larger
lists so above 1000 it uses the builtin input.
Made the spawn_atom use a list subtraction instead of a double list,
it's lighter on memory and processing.

code/game/objects/structures/urban.dm

@@ -67,8 +67,7 @@
 	name = "[street_name]"
 	desc = "This sign indicates this crossing street is called [street_name]."
 
-/obj/structure/stairs/urban
-	abstract_type = /obj/structure/stairs/urban
+ABSTRACT_TYPE(/obj/structure/stairs/urban)
 	icon = 'icons/obj/structure/urban/ledges.dmi'
 	icon_state = "stairs-single"
 	layer = 2.01
@@ -92,12 +91,11 @@
 	dir = SOUTH
 	bound_height = 64
 
-/obj/structure/stairs/urban/road_ramp
+ABSTRACT_TYPE(/obj/structure/stairs/urban/road_ramp)
 	name = "inclined asphalt ramp"
 	desc = "A solid asphalt ramp to allow your vehicle to traverse inclines with ease."
 	icon_state = "road-ramp-center"
 	layer = 2.02
-	abstract_type = /obj/structure/stairs/urban/road_ramp
 
 /obj/structure/stairs/urban/road_ramp/right
 	dir = EAST