Update .npmignore to include generated bundle file

Summary:
to: mkonicek grabbou

Fixes #12183
Fixes #12196

This PR is a followup from #12185 which added this file to the `.gitignore`, but was still not enough because NPM will only use `.npmignore` since one is defined
Closes https://github.com/facebook/react-native/pull/12197

Differential Revision: D4508650

Pulled By: mkonicek

fbshipit-source-id: 9ef5f3308a4637f84630c8953eb26048ee0d2a3c