build: update vue-cli to use purgecss 4.0

Author: Ffloriel

packages/vue-cli-plugin-purgecss/generator/index.js

@@ -1,7 +1,7 @@
 module.exports = (api, options) => {
   api.extendPackage({
     devDependencies: {
-      "@fullhuman/postcss-purgecss": "^3.0.0",
+      "@fullhuman/postcss-purgecss": "^4.0.0",
     },
   });
   api.render("./templates", options);