onPublish (fn) {
    this.#addHook('onPublish', fn)
  }
Run hook if publishing was requested ("$ quasar build -P"),
after Quasar built app for production and the afterBuild
hook (if specified) was executed.

@param {function} fn
  ({ arg, ...}) => ?Promise
     * arg - argument supplied to "--publish"/"-P" parameter
     * quasarConf - quasar.config file config object
     * distDir - folder where distributables were built