registerHandler method
Registers handler for command. Note command cannot have handler if there are options present
Implementation
void registerHandler(SlashCommandHandler handler) {
if (options.any((element) => element.type == CommandOptionType.subCommand || element.type == CommandOptionType.subCommandGroup)) {
throw ArgumentError("Cannot register handler for slash command if command have subcommand or subcommandgroup");
}
this.handler = handler;
}