fix($shared-utils): missing gray-matter dependency (close: #1190)

packages/@vuepress/core/package.json

@@ -44,7 +44,6 @@
     "cross-spawn": "^6.0.5",
     "css-loader": "^0.28.11",
     "file-loader": "^1.1.11",
-    "gray-matter": "^4.0.1",
     "js-yaml": "^3.11.0",
     "koa-connect": "^2.0.1",
     "koa-mount": "^3.0.0",

packages/@vuepress/shared-utils/package.json

@@ -32,7 +32,8 @@
     "fs-extra": "^5.0.0",
     "globby": "^8.0.1",
     "hash-sum": "^1.0.2",
-    "upath": "^1.1.0"
+    "upath": "^1.1.0",
+    "gray-matter": "^4.0.1"
   },
   "author": "ULIVZ <chl814@foxmail.com>",
   "license": "MIT",