Add missing dependency

Closes GH-12.
Closes GH-13.

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

Author: Ayc0

package.json

@@ -48,6 +48,7 @@
     "index.js"
   ],
   "dependencies": {
+    "@types/unist": "^2.0.0",
     "css-selector-parser": "^1.0.0",
     "nth-check": "^2.0.0",
     "unist-util-is": "^5.0.0",