Use `std::variant` with `OfflineRegionDefinition`

[louwers]

Replace Mapbox' variant class with std::variant available with C++17.

[github-actions[bot]]

Bloaty Results (iOS) 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.2% +27.1Ki  +0.2% +32.0Ki    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results-ios/pr-2482-compared-to-main.txt

[github-actions[bot]]

Bloaty Results 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1%  +160Ki  +0.1% +45.7Ki    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-2482-compared-to-main.txt

Compared to d38709084a9865fe0bb8300aec70ebf8243b3d43 (legacy)

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +21% +24.3Mi  +410% +24.5Mi    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results/pr-2482-compared-to-legacy.txt

[github-actions[bot]]

Benchmark Results ⚡

Benchmark                                                     Time             CPU      Time Old      Time New       CPU Old       CPU New
------------------------------------------------------------------------------------------------------------------------------------------
OVERALL_GEOMEAN                                            -0.0052         -0.0053             0             0             0             0

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/benchmark-results/pr-2482-compared-to-main.txt