timezone: Add an option to list all time zones

Userland/Utilities/timezone.cpp

@@ -18,11 +18,19 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     TRY(Core::System::unveil(nullptr, nullptr));
 
     StringView time_zone;
+    bool list_time_zones = false;
 
     Core::ArgsParser args_parser;
+    args_parser.add_option(list_time_zones, "List all available time zones", "list-time-zones", 'l');
     args_parser.add_positional_argument(time_zone, "The time zone to set", "time-zone", Core::ArgsParser::Required::No);
     args_parser.parse(arguments);
 
+    if (list_time_zones) {
+        for (auto time_zone : TimeZone::all_time_zones())
+            outln("{}", time_zone);
+        return 0;
+    }
+
     if (time_zone.is_empty()) {
         outln("{}", TimeZone::current_time_zone());
         return 0;