add audio_manager and code refactoring

default_bus_layout.tres

@@ -0,0 +1,15 @@
+[gd_resource type="AudioBusLayout" format=3 uid="uid://cqvx62pwvvboj"]
+
+[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"