chore: update demo

vue.config.js

@@ -1,6 +1,16 @@
+/* eslint-disable @typescript-eslint/no-var-requires */
+const nested = require("postcss-nested");
+
 module.exports = {
   outputDir: "demo",
   transpileDependencies: ["resize-detector"],
+  css: {
+    loaderOptions: {
+      postcss: {
+        plugins: [nested()]
+      }
+    }
+  },
   chainWebpack: config => {
     config
       .entry("app")