chore: replace node-sass with sass (#768)

Since node-sass has been deprecated [1], we should replace it with sass
instead.

[1]: https://sass-lang.com/blog/libsass-is-deprecated

Author: Xenonym

package.json

@@ -1,7 +1,7 @@
 {
   "devDependencies": {
     "@vuepress/plugin-pwa": "^1.5.4",
-    "node-sass": "^4.13.1",
+    "sass": "^1.32.0",
     "sass-loader": "^8.0.2",
     "vuepress": "^1.5.4"
   },