Set explicit package files

Author: mxmason

package.json

@@ -3,6 +3,10 @@
   "version": "0.0.12",
   "description": "Check for accessibility errors on critical pages of your Netlify website.",
   "main": "lib/index.js",
+  "files": [
+    "lib/**/*",
+    "manifest.yml"
+  ],
   "dependencies": {
     "pa11y": "^6.0.1",
     "path-type": "^4.0.0",