maploader / overmaps update - struct system, overmaps location bindings, misc backend improvements (#6403)

tl;dr level collections that can easily be logically restructured/moved
as necessary

will eventually be used for radio and overmaps location resolution.

this is the last part of
https://github.com/Citadel-Station-13/Citadel-Station-13-RP/pull/4841
that has not yet been integrated.
this will be a draft for a while.

---------

Co-authored-by: LetterN <24603524+LetterN@users.noreply.github.com>

code/modules/asset_cache/assets/json/MapSystem.dm

@@ -0,0 +1,26 @@
+//* This file is explicitly licensed under the MIT license. *//
+//* Copyright (c) 2024 Citadel Station Developers           *//
+
+/**
+ * Contains:
+ *
+ * * map traits
+ * * map attributes
+ */
+/datum/asset_pack/json/MapSystem
+	name = "MapSystem"
+
+/datum/asset_pack/json/MapSystem/generate()
+	. = list()
+
+	var/list/assembled_traits = list()
+	for(var/datum/map_level_trait/map_level_trait_path as anything in subtypesof(/datum/map_level_trait))
+		var/datum/map_level_trait/trait = new map_level_trait_path
+		assembled_traits[initial(map_level_trait_path.id)] = trait.ui_serialize()
+	.["keyedLevelTraits"] = assembled_traits
+
+	var/list/assembled_attributes = list()
+	for(var/datum/map_level_attribute/map_level_attribute_path as anything in subtypesof(/datum/map_level_attribute))
+		var/datum/map_level_attribute/attribute = new map_level_attribute_path
+		assembled_attributes[initial(map_level_attribute_path.id)] = attribute.ui_serialize()
+	.["keyedLevelAttributes"] = assembled_attributes