Add region-based VAT rates support for Spain

[Copilot]

This PR implements region-based VAT rates for Spain as requested in the issue. Spanish regions Ceuta, Las Palmas, Tenerife, and Melilla now have 0% VAT rate, while other regions maintain the standard 21% rate.

Changes Made

Core Implementation

• Extended Party class with optional region_code parameter for backward compatibility
• Enhanced EsVatRules to handle regional VAT calculations:
  • 0% VAT for special regions: Ceuta (CE), Las Palmas (GC), Tenerife (TF), Melilla (ML)
  • 21% VAT for all other Spanish regions (4% for ebooks in non-special regions)
  • Updated both get_sale_to_country_vat_charge and get_sale_from_country_vat_charge methods

Usage Examples

from pyvat import get_sale_vat_charge, ItemType, Party
import datetime

# Standard Spain (21% VAT)
buyer = Party('ES', False)
seller = Party('DE', True)
vat_charge = get_sale_vat_charge(
    datetime.date(2024, 1, 1),
    ItemType.generic_electronic_service,
    buyer, seller
)
print(vat_charge.rate)  # 21

# Spain with Tenerife region (0% VAT)
buyer = Party('ES', False, 'TF')
vat_charge = get_sale_vat_charge(
    datetime.date(2024, 1, 1),
    ItemType.generic_electronic_service,
    buyer, seller
)
print(vat_charge.rate)  # 0

Testing

• Added comprehensive test suite in tests/test_spanish_regions.py
• 8 new test cases covering all scenarios:
  • Special regions (CE, GC, TF, ML) return 0% VAT
  • Regular regions return standard Spanish VAT rates
  • Ebooks in special regions return 0% VAT (overriding the usual 4% rate)
  • Domestic and cross-border transactions work correctly
  • Backward compatibility maintained

Backward Compatibility

✅ Fully backward compatible - existing code continues to work unchanged:

• Party('ES', False) still works as before (21% VAT)
• All existing VAT calculations remain unaffected
• Region code is optional and defaults to None

Key Features

• Only Spain has region-specific VAT rates (as requested)
• Special regions (Ceuta, Las Palmas, Tenerife, Melilla) have 0% VAT
• All other regions maintain standard Spanish VAT rates
• Ebooks follow the same regional rules (0% in special regions, 4% elsewhere)

Fixes #5.

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