toJson abstract method

Convert to map (useful for JSON serialization).

This method creates a Map representation of the locale, which is particularly useful for JSON serialization, database storage, or API communication.

Returns a Map with keys 'language', 'country', and 'variant'. Country and variant may be null if not specified.

Example:

final locale = Locale('en', 'US', 'POSIX');
final map = locale.toJson();
print(map);
// Output: {language: en, country: US, variant: POSIX}

// Convert to JSON
import 'dart:convert';
final json = jsonEncode(map);
print(json); // Output: {"language":"en","country":"US","variant":"POSIX"}