refactors asset subsystem (#6459)

refactors the asset subsystem to make it much more managed

adds support for 'deferred' assets for tgui, which will be later used
for things like tgui character setup and admin panels.
deferred means that it'll be 'sent in a timely manner after the ui
opens', as opposed to 'sent before the ui opens'

---------

Co-authored-by: silicons <no@you.cat>

code/modules/modular_computers/file_system/programs/generic/game.dm

@@ -82,10 +82,9 @@
  * UI assets define a list of asset datums to be sent with the UI.
  * In this case, it's a bunch of cute enemy sprites.
  */
-/datum/computer_file/program/game/ui_assets(mob/user)
-	return list(
-		get_asset_datum(/datum/asset/simple/arcade),
-	)
+/datum/computer_file/program/game/ui_asset_injection(datum/tgui/ui, list/immediate, list/deferred)
+	immediate += /datum/asset_pack/simple/arcade
+	return ..()
 
 /**
  * This provides all of the relevant data to the UI in a list().