Materials Repath (#22388)

Soft-ish port of https://github.com/NebulaSS13/Nebula/pull/540. Except
we call them singletons.

Repaths all materials as singletons instead of datums, and replaces
material defines from strings to paths so that we can just run
GET_SINGLETON instead of needing to use SSMaterials. This is Step One.

This PR has no player-facing changes.

changes:
  - refactor: "Repaths /material to /singleton/material."
- refactor: "Replaces all material string defines to path defines,
replacing SSmaterials procs w/ GET_SINGLETON instead."
- refactor: "Removes all material var edited objects from all maps,
adding new presets where necessary."
- refactor: "Updates recipes unit test to run all recipes against all
material singletons."

---------

Signed-off-by: Batrachophreno <Batrochophreno@gmail.com>
Co-authored-by: kano-dot <bhutanlikanoxy@gmail.com>

code/game/objects/structures/railing.dm

@@ -66,7 +66,7 @@
 /obj/structure/railing/mapped/no_density/low
 	icon_state = "railing0-0"
 
-/obj/structure/railing/New(var/newloc, var/material_key = DEFAULT_WALL_MATERIAL)
+/obj/structure/railing/New(var/newloc, var/material_key = MATERIAL_STEEL)
 	material = material_key // Converted to datum in initialize().
 	..(newloc)
 
@@ -81,7 +81,7 @@
 
 	if(!non_material_object)
 		if(!isnull(material) && !istype(material))
-			material = SSmaterials.get_material_by_name(material)
+			material = GET_SINGLETON(material)
 		if(!istype(material))
 			return INITIALIZE_HINT_QDEL