inject elements

[dweedon-stripe]

Summary & motivation

• inject Elements with injectStripe so that users will be able to call elements.getElement().
• update the documentation with confirmCardPayment and confirmCardSetup examples, remove the deprecated handle* docs.
• support both the new and the old signature for createPaymentMethod, move all the docs and examples to use the new signature.
• update the types
• some misc. readme improvements

Testing & documentation

I wrote unit tests. I updated the demos and made sure they worked.

[cweiss-stripe]

@dweedon-stripe looks like you need to run prettier.

Otherwise the changes look good to me!