feat($core): optional callback when socket connection is ready under dev.

Author: ulivz

packages/@vuepress/core/lib/node/App.js

@@ -451,7 +451,7 @@ module.exports = class App {
    * @api public
    */
 
-  async dev () {
+  async dev (callback) {
     this.isProd = false
     this.devProcess = new DevProcess(this)
     await this.devProcess.process()
@@ -462,7 +462,7 @@ module.exports = class App {
         this.process()
       })
       .createServer()
-      .listen()
+      .listen(callback)
   }
 
   /**

packages/@vuepress/core/lib/node/dev/index.js

@@ -250,7 +250,7 @@ module.exports = class DevProcess extends EventEmitter {
 
   listen (callback) {
     this.server.listen(this.port, this.host, (err) => {
-      if (callback) {
+      if (typeof callback === 'function') {
         callback(err)
       }
     })