osc: kitty notifications: implement s=silent

This implements part of the new 's' (sound) parameter; the 'silent'
value. When s=silent, we set the ${muted} template argument to
"true". It is intended to set the 'suppress-sound' hint:

    notify-send --hint BOOLEAN:suppress-sound:${muted}

notify.h

@@ -52,6 +52,8 @@ struct notification {
     bool report_activated;  /* OSC-99: report notification activation to client */
     bool report_closed;     /* OSC-99: report notification closed to client */
 
+    bool muted;            /* Explicitly mute the notification */
+
     /*
      * Used internally by notify
      */