fix($shared-utils): Slugify em/en dash in urls (#2174)

Author: wdporter

packages/@vuepress/shared-utils/src/slugify.ts

@@ -4,7 +4,7 @@ import { remove as removeDiacritics } from 'diacritics'
 
 // eslint-disable-next-line no-control-regex
 const rControl = /[\u0000-\u001f]/g
-const rSpecial = /[\s~`!@#$%^&*()\-_+=[\]{}|\\;:"'“”‘’<>,.?/]+/g
+const rSpecial = /[\s~`!@#$%^&*()\-_+=[\]{}|\\;:"'“”‘’–—<>,.?/]+/g
 
 export = function slugify (str: string): string {
   return removeDiacritics(str)