Packages: Install back-to-top plugin

Author: ramiy

package-lock.json

@@ -14,6 +14,7 @@
   },
   "license": "MIT",
   "devDependencies": {
+    "@vuepress/plugin-back-to-top": "^1.9.7",
     "@vuepress/plugin-google-analytics": "^1.9.7",
     "@vuepress/plugin-pwa": "^1.9.7",
     "vuepress": "^1.9.7",