Adds a test-mode developer assistant in Elements with Intents

What’s new

A test-mode developer assistant is now automatically rendered in Elements.

Impact

This change streamlines development by adding a test‑mode developer assistant to Elements. The assistant provides payment method autofill, customer location mocking, and integration optimizations. You can use the assistant to test your integrations and optimize your checkout flow. See Stripe.js test mode assistant for more information.