fix(isVAT): accept Spanish digit controls

[ANONYMOUSZED-beep]

Fixes #2846.

The Spanish VAT matcher currently requires a trailing letter, so it rejects valid legal-entity NIFs whose control character is a digit. It also accepts unassigned entity prefixes and mismatched prefix/control forms.

This change groups Spanish legal-entity prefixes by their permitted control type:

• digit: A, B, E, H
• letter: N, P, Q, R, S, W
• digit or letter: C, D, F, G, J, U, V

Regression coverage includes prefixed and unprefixed digit-control values, letter-control and dual-control forms, plus mismatched controls, unassigned prefixes, and malformed length.

References:

• AEAT: Composición del NIF de personas jurídicas y entidades
• Orden EHA/451/2008

Validation:

• npx mocha --require @babel/register --reporter dot --recursive --grep "validate VAT numbers"
• npx eslint src/lib/isVAT.js test/validators.test.js
• npm test — 323 passing; build and full ESLint included

AI assistance was used to prepare and validate this focused change; the final diff and test results were reviewed before submission.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (not applicable; no API contract change)
• Tests written
• References provided

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (cdb7daf) to head (e264ea3).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2849   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2599      2599           
  Branches       658       658           
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  Hits          2599      2599           

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