Don't use slugger singleton (github#17579)

Author: JasonEtco

lib/render-content/plugins/use-english-headings.js

@@ -2,7 +2,6 @@ const GithubSlugger = require('github-slugger')
 const Entities = require('html-entities').XmlEntities
 const toString = require('hast-util-to-string')
 const visit = require('unist-util-visit')
-const slugger = new GithubSlugger()
 const entities = new Entities()
 
 const matcher = node => (
@@ -20,7 +19,7 @@ module.exports = function useEnglishHeadings ({ englishHeadings }) {
       // find English heading in the collection
       const englishHeading = englishHeadings[entities.encode(text)]
       // get English slug
-      const englishSlug = slugger.slug(englishHeading)
+      const englishSlug = GithubSlugger.slug(englishHeading)
       // use English slug for heading ID and link
       node.properties.id = englishSlug
     })