Display a notice if taxes vary by customer's shipping address

[wjrosa]

Fixes #3521

Changes proposed in this Pull Request:

This PR adds a new notice to the Express Checkout Element informing customers when rates may differ due to taxes being based on their billing address.

The notice will be shown, and the checkout process will be delayed for 700ms. Reasons for that:

• We cannot display an alert message because it blocks the default ECE process
• The delay cannot be higher than 1s due to Stripe JS limitations (it times out after 1s)

Testing instructions

• Checkout and build this branch on your test environment (fix/display-notice-when-taxes-may-differ-on-ece)
• Enable ECE. You can do this by hardcoding is_stripe_ece_enabled to true
• As a merchant, connect your Stripe account
• Enable the Express Checkout methods (Link, Google Pay, Apple Pay)
• Enable tax rates and calculations (wp-admin/admin.php?page=wc-settings&tab=general)
• Set taxes based on the customer's shipping address (wp-admin/admin.php?page=wc-settings&tab=tax)
• Add some shipping zones
• As a shopper, try to purchase any physical product by any means
• Confirm that you can complete the process without seeing the notice
• Set taxes based on the customer's billing address (wp-admin/admin.php?page=wc-settings&tab=tax)
• Repeat the checkout process
• Confirm that now you can see the new notice:

---

• [x] 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)

[annemirasol]

@wjrosa Thanks for working on this!

✅ I see the notice for product and shortcode checkout. ❌ I don't see the notice for block checkout and block cart.

I set my taxes to be based on billing address and my ECE is enabled.

[wjrosa]

@wjrosa Thanks for working on this!

✅ I see the notice for product and shortcode checkout. ❌ I don't see the notice for block checkout and block cart.

I set my taxes to be based on billing address and my ECE is enabled.

Sorry about that, Anne! I just forgot about the block checkout and cart 🤦 . Can you check again, please?