Add audio bus configuration for various sound effects and update main scene reference

default_bus_layout.tres

@@ -0,0 +1,15 @@
+[gd_resource type="AudioBusLayout" format=3 uid="uid://b2d2n7cffo3bi"]
+
+[resource]
+bus/1/name = &"music"
+bus/1/solo = false
+bus/1/mute = false
+bus/1/bypass_fx = false
+bus/1/volume_db = 0.0
+bus/1/send = &"Master"
+bus/2/name = &"sfx"
+bus/2/solo = false
+bus/2/mute = false
+bus/2/bypass_fx = false
+bus/2/volume_db = 0.0
+bus/2/send = &"Master"