build: migrate demo from webpack to Vite (#832)

vite.config.ts

@@ -0,0 +1,17 @@
+import { defineConfig } from "vite";
+import vue from "@vitejs/plugin-vue";
+import postcssNested from "postcss-nested";
+
+// https://vite.dev/config/
+export default defineConfig({
+  plugins: [vue()],
+  root: "./demo",
+  server: {
+    host: true
+  },
+  css: {
+    postcss: {
+      plugins: [postcssNested()]
+    }
+  }
+});