feat(cli): use clean-css to minify css

packages/vant-cli/package.json

@@ -27,7 +27,6 @@
     "vue-template-compiler": "^2.6.11"
   },
   "devDependencies": {
-    "@types/csso": "^3.5.1",
     "@types/eslint": "^6.1.3",
     "@types/fs-extra": "^8.0.1",
     "@types/html-webpack-plugin": "^3.2.1",
@@ -66,12 +65,12 @@
     "babel-loader": "^8.0.6",
     "babel-plugin-import": "^1.13.0",
     "chokidar": "^3.3.1",
+    "clean-css": "^4.2.1",
     "codecov": "^3.6.1",
     "commander": "^4.0.1",
     "conventional-changelog": "^3.1.18",
     "cross-env": "^6.0.3",
     "css-loader": "^3.4.0",
-    "csso": "^4.0.2",
     "eslint": "^6.7.2",
     "find-babel-config": "^1.2.0",
     "gh-pages": "2.0.1",