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/plasticflaps.dm

@@ -23,7 +23,7 @@
 
 /obj/structure/plasticflaps/Initialize()
 	. = ..()
-	material = SSmaterials.get_material_by_name(MATERIAL_PLASTIC)
+	material = GET_SINGLETON(MATERIAL_PLASTIC)
 	update_icon()
 
 /obj/structure/plasticflaps/update_icon()