add osmd.PlaybackManager getter

src/OpenSheetMusicDisplay/OpenSheetMusicDisplay.ts

@@ -104,6 +104,10 @@ export class OpenSheetMusicDisplay {
         }
     }
 
+    public get PlaybackManager(): PlaybackManager {
+        return this.renderingManager?.PlaybackManager;
+    }
+
     private isUrl (url: string): boolean {
         if (url.length < 2083) {
             return true;