fix: add select2 to whitelisted scripts

[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:

This PR adds Select2 to the allowed scripts in the modal checkout.

How to test the changes in this Pull Request:

1. Run through a modal checkout with a physical product.
2. Note the appearance of the country/state dropdowns:

3. Apply this PR.
4. Confirm that the dropdowns are styled correctly:

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?

[miguelpeixe]

Thank you for fixing that!

[laurelfulford]

Thanks for the review! 🎉