Update Express Payment Button previews on the Block Checkout and Cart edit pages

[james-allan]

Fixes #3541

Changes proposed in this Pull Request:

This PR updates the Express Payment button previews displayed on the edit Block checkout & cart pages for Google Pay and Apple Pay.

Before	After

These buttons previously just used image elements with SVGs and were different sizes, and spacing was inconsistent. This new approach sets up a base component we can use going forward to mimic what the buttons will look like in a consistant way.

Testing instructions

1. Enable Express payment buttons in your Stripe settings.
2. Checkout this branch.
3. Run npm run build.
4. Edit a Checkout Block and or Cart block page and note the button previews.

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• [ ] Covered with tests (or have a good reason not to test in description ☝️)
• [x] Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• [ ] Tested on mobile (or does not apply)

Post merge

• [ ] Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• [ ] Added the needs docs label (or does not apply)
• [ ] Included this PR in the Release Thread scope (or does not apply)