All Submissions:

• Have you followed the WooCommerce Contributing guideline?
• Does your code follow the WordPress' 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:

The PR adds Denmark postcode validation according to https://en.wikipedia.org/wiki/Postal_codes_in_Denmark. Essentially it's a minor modification of https://rgxdb.com/r/335G4K1S with an optional DB prefix.

As the Denmark postcode can contain DK- prefix I restored it in the wc_format_postcode function (as I can see LV has already).

Regarding the unit tests, I was a bit confused because the current \WC_Validation::is_postcode tests are included just in \WC_Tests_Validation::data_provider_test_is_postcode method. It should not be touched according to README.md. Should I add a new testing class to plugins/woocommerce/tests/php/includes?

• [] This PR is a very minor change/addition and does not require testing instructions (if checked you can ignore/remove the next section).

How to test the changes in this Pull Request:

1. Go to the checkout page to finish your order.
2. Select Denmark as the billing (or shipping) country.
3. Paste the postcode in the wrong format (like DK-3900).
4. Try to complete the purchase.
5. You get a notification about the wrong Postal code.
6. Change the postal code to the valid one (like 1448 or DK-1448).
7. You are able to complete the purchase.

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 created a changelog file for each project being changed, ie pnpm --filter=<project> changelog add?

FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.