Add new card fields component

[gregjopa]

This PR adds the new Checkout purchase example for card-fields. The example will live under the /advanced-integration/beta folder structure for now.

Notable changes:

1. Remove ejs since we want to make it as easy as possible to teach. Similar to how the Standard Integration example for Buttons does not use any templating language.
2. Use the client/server folder structure for code organization.
3. Use default linter and prettier settings (except for html because prettier is too opinionated with closing tags)

The vault example can be added in a future PR.

Steps to test

1. Checkout this upstream branch new-card-fields
2. cd advanced-integration/beta
3. Add a .env file and use the format in .env.example
4. npm install
5. npm start
6. Use a test sandbox credit card from https://developer.paypal.com/api/rest/sandbox/card-testing/#link-creditcardgeneratorfortesting