Fix misclassified dependencies

Author: wooorm

package.json

@@ -35,13 +35,14 @@
     "index.js"
   ],
   "dependencies": {
+    "@types/mdast": "^3.0.0",
+    "mdast-util-to-markdown": "^1.3.0",
     "micromark-extension-frontmatter": "^1.0.0"
   },
   "devDependencies": {
     "@types/tape": "^4.0.0",
     "c8": "^7.0.0",
     "mdast-util-from-markdown": "^1.0.0",
-    "mdast-util-to-markdown": "^1.0.0",
     "prettier": "^2.0.0",
     "remark-cli": "^10.0.0",
     "remark-preset-wooorm": "^9.0.0",