Issue #3536839: Add default weights to payment information checkout pane elements
Add #weight properties to payment information checkout pane elements to simplify developer customization and element reordering:
- Payment method radios: weight -10 (first element)
- Add payment method form: weight 0 (middle element)
- Billing information form: weight 10 (last element)
This allows developers to easily insert custom elements at intermediate weights like -5, 5, 15, etc. to achieve desired element ordering in checkout flows.
Closes #3536839