CLI entry point for g18n-dev tools

Commands

gleam run generate          # Generate from flat JSON files  
gleam run generate --nested # Generate from nested JSON files
gleam run generate -- po    # Generate from PO files
gleam run report            # Show translation report
gleam run report --primary <locale> # Set primary locale for comparison
gleam run help              # Show help
gleam run                   # Show help (default)