Fix LU postal code regex to allow zero as first digit

[o-benz]

What are you trying to accomplish?

Relates to shop/issues-addresses#647

ONLY MERGE AFTER #415

Postal codes starting with 0 are valid in Luxembourg and are used for packstation/parcel locker services (e.g., 0692). Updated regex from [1-9]\d{3} to \d{4} to allow these codes.

What approach did you choose and why?

I updated the LU zip_regex (and matching partial_zip_regex).

What should reviewers focus on?

The updated regex and the new regression tests, especially that valid formats still pass and bad ones still fail.

The impact of these changes

Valid LU postcodes that were previously rejected will now be accepted. Valid LU postcodes should remain unaffected.

Testing

Added test/worldwide/regions/lu_test.rb covering valid LU outward formats and rejecting the invalid cases.

Next Steps

• Release this worldwide version (1.21.1), that cover both this patch and my previous unrelated one (see Fix GB postcode regex to reject invalid outward formats #415) and bump it in country_db, release new country_db version and finally bump it in checkout.

Checklist

• I have added a CHANGELOG entry for this change (or determined that it isn't needed)