Add a CLI interface for Fast Flight

[swiftlysingh]

Summary

• Introduced a production-ready flights-cli command that wraps get_flights, supports multi-segment itineraries, seat classes, passenger counts, fetch modes, HTML/JS data-source switching, JSON output, and a --best-only toggle—built so I could drive the scraper directly from the terminal.
• Added the CLI entry point wiring in pyproject.toml/setup.py and pulled in the missing typing_extensions dependency so installations surface the flights-cli tool.
• Authored docs/cli.md documenting installation, argument formats (including passenger defaults), examples for one-way/round-trip/multi-city searches, and pointers to existing docs.

Testing

• python -m fast_flights.cli --help
• pip install -e .
• flights-cli --help