Add missing types dependency

Closes GH-21.

Reviewed-by: Remco Haszing <remcohaszing@gmail.com>
Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

package.json

@@ -34,6 +34,7 @@
   ],
   "dependencies": {
     "@types/hast": "^3.0.0",
+    "@types/unist": "^3.0.0",
     "@ungap/structured-clone": "^1.0.0",
     "hast-util-from-parse5": "^8.0.0",
     "hast-util-to-parse5": "^8.0.0",