pt_BR: support new alphanumeric CNPJ format (Serpro, effective July 2026)

The pt_BR provider's cnpj() and company_id() still generate the old numeric-only CNPJ. Brazil's new alphanumeric CNPJ format is not supported. The first 12 characters can now be alphanumeric (0-9, A-Z); only the 2 check digits remain numeric.

Steps to reproduce

1. from faker import Faker
2. fake = Faker("pt_BR")
3. fake.cnpj()

Expected behavior

cnpj() should be able to produce alphanumeric CNPJs following the official Serpro algorithm, e.g. 12.ABC.345/01DE-35. Check digits still computed via mod-11 over each character's value (ASCII - 48).

Reference: "Cálculo dos dígitos verificadores de CNPJ alfanumérico"

Actual behavior

cnpj() only ever returns numeric values, e.g. 12.345.678/0001-95. There is no way to generate the new alphanumeric format.