fix(modal-checkout): apply margin to after form rows added via after billing fields hook

[chickenn00dle]

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:

Goes with the following Plugin PR: https://github.com/Automattic/newspack-plugin/pull/3546

This PR fixes an issue where checkboxes (or anything else) added via the woocommerce_after_checkout_billing_form did not have the correct margin.

We fix this by applying our expected margin to any .form-row added to this part of checkout. Note we only target .form-row as this is the expected format of anything added here. Anything added using a nonstandard class will need additional custom CSS to integrate correctly.

NOTE: The fix for the off-centered text and checkbox input live in the plugin PR: https://github.com/Automattic/newspack-plugin/pull/3546

How to test the changes in this Pull Request:

1. As admin add Mailchimp for WooCommerce to your test site and get it connected
2. Ensure the Mailchimp for WooCommerce newsletters checkbox is added at checkout via Mailchimp > Store > Checkout page settings
3. As a reader open modal checkout via any checkout button or donate block
4. On the billing fields screen scroll to the bottom and ensure the added form row has top margin but no bottom margin as pictured above.

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?

[chickenn00dle]

Assigning you @laurelfulford since this is style related and you are probably best to review!