function remarkStringify(options) {
  const self = this

  /**
   * @param {Root} tree
   * @returns {string}
   */
  self.compiler = function (tree) {
    return toMarkdown(tree, {
      ...self.data('settings'),
      ...options,
      // Note: this option is not in the readme.
      // The goal is for it to be set by plugins on `data` instead of being
      // passed by users.
      extensions: self.data('toMarkdownExtensions') || []
    })
  }
}
Add support for serializing to markdown.

@this {Processor<undefined, undefined, undefined, Root, string>}
  Processor instance.
@param {Readonly<Options> | null | undefined} [options]
  Configuration (optional).
@returns {undefined}
  Nothing.