src/core: add an asset manager to the scene object

sonic-radiance.love/core/modules/scenes.lua

@@ -23,10 +23,13 @@
 ]]
 
 local Scene  = Object:extend()
+local Assets = require "core.modules.assets"
 
 function Scene:new()
   self.mouse = {}
   self.mouse.x, self.mouse.y = core.screen:getMousePosition()
+
+  self.assets = Assets()
 end
 
 function Scene:register()