Add missing dependency for types

Closes GH-675.

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

Author: Methuselah96

package.json

@@ -80,6 +80,7 @@
   ],
   "dependencies": {
     "@types/hast": "^2.0.0",
+    "@types/prop-types": "^15.0.0",
     "@types/unist": "^2.0.0",
     "comma-separated-tokens": "^2.0.0",
     "hast-util-whitespace": "^2.0.0",