🤖 chore(package.json): Add devDependency xo.

These changes were automatically generated by a transform whose code can be found at:
  - https://github.com/aureooms/rejuvenate/blob/1fe650dc2407c6c6395828a28777fe8137a3bd2f/src/transforms/deps:add-xo.js
Please contact the author of the transform if you believe there was an error.

Author: a-flying-potato

package.json

@@ -66,7 +66,8 @@
     "fixpack": "4.0.0",
     "np": "7.5.0",
     "power-assert": "1.6.1",
-    "regenerator-runtime": "0.13.7"
+    "regenerator-runtime": "0.13.7",
+    "xo": "0.40.1"
   },
   "ava": {
     "files": [