feat: modal checkout details toggle

[laurelfulford]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Along with Automattic/newspack-blocks#2048, adds a toggle-able section for Transaction Details.

How to test the changes in this Pull Request:

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[laurelfulford]

Closing this in favour of Automattic/newspack-blocks#2049.