How to use the @vuepress/markdown-loader.frontmatterEmitter.on function in @vuepress/markdown-loader

(
      ctx.siteConfig.extraWatchFiles || []
    ).map(file => normalizeWatchFilePath(file, ctx.sourceDir))
  )

  logger.debug('watchFiles', watchFiles)

  // watch config file
  const configWatcher = chokidar.watch(watchFiles, {
    cwd: sourceDir,
    ignoreInitial: true
  })
  configWatcher.on('change', spawnUpdate('change'))

  // also listen for frontmatter changes from markdown files
  frontmatterEmitter.on('update', () => update('frontmatter or headers change'))

  // resolve webpack config
  let config = createClientConfig(ctx)

  config
    .plugin('html')
    // using a fork of html-webpack-plugin to avoid it requiring webpack
    // internals from an incompatible version.
    .use(require('vuepress-html-webpack-plugin'), [{
      template: ctx.devTemplate
    }])

  config
    .plugin('site-data')
    .use(HeadPlugin, [{
      tags: ctx.siteConfig.head || []

watchFrontmatter () {
    frontmatterEmitter.on('update', target => this.handleUpdate('frontmatter', target))
  }