feat: add message event

add event to listen to messages coming from the child's ipc events, partially implementing #1519

lib/monitor/run.js

@@ -133,6 +133,12 @@ function run(options) {
       bus.stdout = child.stdout;
       bus.stderr = child.stderr;
     }
+
+    if (shouldFork) {
+      child.on('message', function (message, sendHandle) {
+        bus.emit('message', message, sendHandle);
+      });
+    }
   }
 
   bus.emit('start');