fix(bg/vat): accept 9-digit numbers whatever their first digit

[kallehauge]

Problem

bg/vat rejects registered Bulgarian VAT numbers whose first digit is not 0, 1, 2, 3 or 9. Vivacom Bulgaria, one of the country's largest telecom operators, is an example:

stdnum.BG.vat.validate('BG831642181');
// { isValid: false, error: InvalidFormat }

BG831642181 is a valid, registered VAT number. Its check digit is correct; validation stops at the first-digit rule before the check digit is computed.

The numbers in this PR were checked in the EU VIES service before it was opened.

The official format has no first-digit rule

• Bulgarian VAT Act (ЗДДС), art. 94(2): the VAT number of a registered company is BG followed by its unified identification code (ЕИК).
• Ordinance No. 1 of 14 February 2007 on the Commercial Register (official PDF, Registry Agency), art. 114(3): the ЕИК "is 9 digits, is assigned at first registration and stays unchanged until deletion". It sets no rule for the first digit.

python-stdnum agrees: its validate() checks the length, that the number is all digits, and the check digit, and no version of stdnum/bg/vat.py since 2012 has had a first-digit rule. python-stdnum 2.2 accepts 831642181. The first-digit rule has been in bg/vat.ts since the module was added, with no source given.

Fix

This PR removes the rule. The check digit still applies to every number.

Input	Before	After
BG831642181	InvalidFormat	valid
BG831642182 (wrong check digit)	InvalidFormat	InvalidChecksum
BG175074752	valid	valid

Tested

• src/bg/vat.spec.ts: 2 new cases; they fail before the change and pass after it.
• Full suite: TZ=UTC npx jest --runInBand passes, 200 suites / 1637 tests.
• npm run lint passes.