[koblas]

Adds validateVat(country, value), next to validatePerson and validateEntity, with the same return shape. It looks up only euVat, which besides the EU member states includes some non-EU countries (AD, CH). A country not in euVat returns { checked: false }.

Fixes #50

Tree shaking

validateVat reads only euVat, so it doesn't pull in the stdnum, personValidators or entityValidators tables. Measured with esbuild (--bundle --minify --format=esm) on an ESM build:

Entry	Before	After
import { BR } from 'stdnum'; BR.cpf.validate()	7,240 B	7,240 B
import { validatePerson } from 'stdnum'	117,941 B	117,941 B
import { validateVat } from 'stdnum'	n/a	39,954 B

This depends on "sideEffects": false in package.json: without it the first entry grows to about 281 KB. The README now says so.

Changes

• src/index.ts: validateVat. It uses toUpperCase() for the country code, like the passport helpers.
• src/index.spec.ts: 7 tests (valid DE, FR and CH VAT numbers; a bad check digit; lowercase codes; an unknown country; US, which is in entityValidators but not euVat). All values come from existing spec fixtures. All 7 fail without the function.
• README.md: a "Validating by country code" section covering validatePerson, validateEntity and validateVat (the first two weren't documented), and a note on sideEffects.

Not verified

• Sizes were measured with esbuild only, not webpack or Rollup.
• npx prettier --check README.md fails because of the passport table added in feat(passport): add passport number validators #175, which was already unformatted. The new section is clean; I didn't reformat the table to keep the diff small.