chore: add mising deps

Author: ulivz

packages/@vuepress/core/package.json

@@ -67,7 +67,8 @@
     "webpack-chain": "^4.6.0",
     "webpack-merge": "^4.1.2",
     "webpack-serve": "^1.0.2",
-    "webpackbar": "^2.6.1"
+    "webpackbar": "^2.6.1",
+    "markdown-it-chain": "^1.2.0"
   },
   "engines": {
     "node": ">=8"