chore: add unified to devDependencies (#5839)

Co-authored-by: undefined <undefined>

Author: izhouteng

package.json

@@ -246,6 +246,7 @@
     "ts-loader": "^9.1.0",
     "typescript": "~4.5.2",
     "umi-request": "^1.3.5",
+    "unified": "9.2.2",
     "url-loader": "^3.0.0",
     "vite": "^3.0.0",
     "vue": "^3.2.0",