worker: improve JS-side debugging

Do not print debug messages that indicate that a stdio stream
has drained; because `util.debuglog()` uses `console.log`, which
in turn uses stdio streams, this would otherwise have lead to an
endless loop.

PR-URL: #25312
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/worker/io.js

@@ -62,7 +62,8 @@ MessagePort.prototype.unref = MessagePortPrototype.unref;
 // .onmessage events, and a function used for sending data to a MessagePort
 // in some other thread.
 MessagePort.prototype[kOnMessageListener] = function onmessage(payload) {
-  debug(`[${threadId}] received message`, payload);
+  if (payload.type !== messageTypes.STDIO_WANTS_MORE_DATA)
+    debug(`[${threadId}] received message`, payload);
   // Emit the deserialized object to userland.
   this.emit('message', payload);
 };