fix(modal-checkout): disable Continue button until JS is loaded

[adekbadek]

All Submissions:

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

Changes proposed in this Pull Request:

In modal checkout, the "Continue" button is enabled by default, but it will have no effect until JS is loaded. This PR makes it disabled until the relevant JS is loaded.

How to test the changes in this Pull Request:

1. On trunk, initiate a donation (only click the "Donate" button) and visit /checkout/?modal_checkout=1
2. In the Network tab of devtools, block the request to modalCheckout.js
3. Refresh the page, observe the button is enabled, but nothing happens on click
4. Switch to this branch, reload the page, observe the button is disabled
5. Disable network request blocking, refresh the page, observe the button is enabled as soon as JS loads

Other information:

• [x] 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?
• [x] Have you successfully ran tests with your changes locally?

[matticbot]

:tada: This PR is included in version 4.3.5-alpha.1 :tada:

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

Your semantic-release bot :package::rocket:

[matticbot]

:tada: This PR is included in version 4.3.5 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: