Add get_admin1_codes() for first-level administrative divisions

[cvl01]

Summary

Adds support for the GeoNames first-level administrative division dataset (admin1CodesASCII.txt), exposed via a new GeonamesCache.get_admin1_codes() method.

Design

The returned dictionary is keyed by the composite code <countrycode>.<admin1code>, e. g. US.CA, matching the format used in the GeoNames source file. This is the headline benefit of the composite key: city records store countrycode and admin1code as separate fields, so their admin1 references are directly resolvable:

import geonamescache

gc = geonamescache.GeonamesCache()
city = gc.get_cities()['5368361']  # Los Angeles
admin1 = gc.get_admin1_codes()[f"{city['countrycode']}.{city['admin1code']}"]
print(admin1['name'])  # California

Each record contains name, asciiname and geonameid.

Changes

• bin/download_data.py: download admin1CodesASCII.txt
• bin/admin1.py: new generator producing admin1.json
• Makefile: run the new generator in the json target
• geonamescache/types.py: new Admin1 TypedDict and Admin1CodeStr key alias
• geonamescache/__init__.py: new get_admin1_codes() method
• Tests, README and CHANGELOG updated

Verification

• pytest: all new and existing tests pass (the new tests cover key lookup, dataset size and resolving a city's admin1 reference)
• ruff check and mypy geonamescache tests pass