0.3.0

Updated Swift Tools version to 6.0 (stil using language version 5).

Breaking Changes

• ARTCC: ICAO ID is now optional.

API Changes

• ISO-Latin1 encoding is used when parsing distribution text files.
• Moved from NSLocalizedString to string catalogs.
• Runway: Added a gradient estimation method that uses the base and reciprocal
  elevations (when known). You can use this is a stopgap until the FAA resumes
  distributing runway gradient data.
• Dependency updates.