Adds new debug verb for highlighting roundstart ATs (#26492)

* Adds new debug verb for highlighting roundstart ATs

Debug -> Debug Verbs enable
then
Mapping -> Show roundstart AT markers

Will highlight AT turfs, for easier location and comprehension rather
than looking at the list of coords. List of coords still works fine
though.

code/game/objects/effects/misc.dm

@@ -30,4 +30,23 @@
 	icon_state = "white"
 	plane = LIGHTING_PLANE
 	layer = LIGHTING_LAYER
-	blend_mode = BLEND_ADD
+	blend_mode = BLEND_ADD
+
+/obj/effect/abstract/marker
+	name = "marker"
+	icon = 'icons/effects/effects.dmi'
+	anchored = TRUE
+	icon_state = "wave3"
+	layer = RIPPLE_LAYER
+
+/obj/effect/abstract/marker/Initialize(mapload)
+	. = ..()
+	GLOB.all_abstract_markers += src
+
+/obj/effect/abstract/marker/Destroy()
+	GLOB.all_abstract_markers -= src
+	. = ..()
+
+/obj/effect/abstract/marker/at
+	name = "active turf marker"
+