Dart 3 support, enhanced coordinate data structures and other features.

Key features

• Moving to Dart 3.6 from Dart 2.17, code base refactored to use new language features, but keeping breaking changes as minimum as possible
• Coordinate / position data structure enhancements (cartesian local coordinates, local tangent planes, geographic orientation etc.)
• Other features to be planned later

NOTE: originally the release 2.0.0 was waiting macro support, but that canceled, there's no point on waiting new Dart future SDK versions.

Dart SDK requirement: at least Dart 3.4+ (records, patterns, extension types, etc.), starting for geobase releases 2.0.0. However 1.x versions of these packages continue to require Dart 2.17 (in pubspec.yaml it's specified as sdk: '>=2.17.0 <4.0.0'.