Add missing types dependency

Closes GH-4.

Reviewed-by: Titus Wormer <[email protected]>

Author: alvinleung1996

package.json

@@ -37,6 +37,7 @@
   ],
   "dependencies": {
     "@types/mdast": "^3.0.0",
+    "@types/unist": "^2.0.0",
     "mdast-util-to-markdown": "^1.0.0",
     "parse-entities": "^3.0.0",
     "stringify-entities": "^4.0.0",