feat: add support for alphanumeric CNPJ

[gabeins]

Implements support for the new alphanumeric CNPJ format being introduced in July 2026 by Brazil's Federal Revenue Service, while maintaining full backward compatibility with numeric CNPJs.

Changes:

• Add alphanumeric validation with modulo 11 check digit algorithm
• Add strict mode option to validate() for uppercase enforcement
• Update format() to handle both numeric and alphanumeric CNPJs
• Update generate() with optional format parameter (numeric/alphanumeric)
• Add comprehensive test coverage (24 tests, all passing)
• Update documentation with examples and API details

The validate() function now accepts an optional second parameter: { strict: boolean } - rejects lowercase letters when true

The generate() function now accepts an optional parameter: { format: 'numeric' | 'alphanumeric' } - defaults to 'numeric'

All existing code continues to work without modifications.

Closes #33.