feat: add Brazilian CPF validator

[easedu]

Description

I'd like to contribute a validator for Brazilian CPF (Cadastro de Pessoa Física),
Brazil's individual taxpayer registry identification similar to SSN in the US.

Rationale

• CPF is widely used in Brazilian applications
• Complements the existing CNPJ validator (which I recently contributed in #YOUR_PR_NUMBER)
• High demand from Brazilian developer community

Proposed Implementation

• Add isCPF() function following the same pattern as existing tax ID validators
• Validation algorithm: Module 11 with check digits (similar to CNPJ)
• Format: XXX.XXX.XXX-XX (11 digits total, last 2 are check digits)
• Accept formatted (with dots/dash) and unformatted inputs

References

• Used by millions of Brazilian citizens and companies

I'm ready to implement this if the maintainers approve. Happy to discuss any
implementation details or alternative approaches.