Make npm package private.

This project has npm's `package.json`, but it is not published on npm and is meant to be private. Mark it as such.
https://docs.npmjs.com/files/package.json#private

Author: grant

package.json

@@ -8,6 +8,7 @@
   },
   "author": "Eric Koleda <ekoleda+devrel@googlers.com>",
   "license": "Apache-2.0",
+  "private": true,
   "bugs": {
     "url": "https://github.com/googlesamples/apps-script-oauth2/issues"
   },