Add channel selection

src/jsMain/kotlin/nl/astraeus/vst/chip/audio/AudioModule.kt

@@ -54,6 +54,9 @@ abstract class AudioNode(
   abstract fun onMessage(message: MessageEvent)
 
   open fun postMessage(msg: Any) {
+    if (port == null) {
+      console.log("postMessage port is NULL!")
+    }
     port?.postMessage(msg)
   }