Web PaymentElement to support SetupIntent

[clintdoriot]

Is your feature request related to a problem? (please describe)

I'm trying to use the PaymentElement on web to allow users to set up their payment method (CC, Apple, Google) in advance. I can generate the setup intent using my server API, and pass the client secret into the PaymentElement.

However, the only option I'm seeing to submit the PaymentElement is WebStripe.instance.confirmPaymentElement(), which assumes it has a payment intent instead of a setup intent. Submitting that results in the error:

IntegrationError: Your code called confirmPayment() but you passed a client_secret associated with a SetupIntent. Did you mean to call confirmSetup() instead?

Describe the solution you'd like
I would like to be able to setup the PaymentElement with a SetupIntent, and be able to trigger the confirmSetup function

[clintdoriot]

Seems like this PR is meant to address this issue: #1881

For others in a similar situation, the workaround I've found is to use the API to set up an incomplete subscription, grab the payment intent from the invoice, and populate the PaymentElement that way. Its not great (if they change subscription tiers, I have to start over, generating a new subscription and forcing the user to re-enter payment info), but it works for now.