r? @lfades
cc @timothyis @dayhaysoos @ChrisBrownie55
* Add [use-shopping-cart](https://github.com/dayhaysoos/use-shopping-cart) example to stripe sample
* Refactor how we instantiate Stripe across the different pages with a `getStripe` singleton pattern.
- Update to latest stripe-node version and pin the exact version so the Types won't go out of sync
- Add the env var changes for next 9.4.4 (https://nextjs.org/blog/next-9-4#new-environment-variables-support)
- Remove `next.config.js` as no longer needed.
- Add the Deploy to Vercel button
