Add alternative side-effect free import path

[christopher-stripe]

Summary & motivation

Adds an alternative path for importing loadStripe, which delays loading the Stripe.js script until loadStripe is called:

import {loadStripe} from '@stripe/stripe-js/pure';

Testing & documentation

• Added 2 tests for the side-effect in pure.test.ts
• Configured the loadStripe tests to run against both the default module and pure module
• Added section to README documenting the import path
• Tested manually with a create-react-app app

[hofman-stripe]

Looks good, just some small nits

[hofman-stripe]

Couple issues with the tests I had missed originally.