Reduce embedded and express to separate JS files

[mntzrr]

Some logic in checkout express is not needed for the embedded checkout. This shared logic has also caused issues with the UI due to assumptions that are true for checkout express, but not embedded checkout.

• separates the logic for easier maintenance.
• reduce file size.
• check if the Dintero iframe exists before trying to mount the embedded checkout.
• [embedded checkout] all checkout fields will now be blocked while the payment is being processed.

https://app.clickup.com/t/8698t4dj5

[MichaelBengtsson]

@mntzrr Looks good, but i want to test this a bit before we merge it. I will try to get that done today.