commandsGroup(heading) {
    if (heading === undefined) return this._defaultCommandGroup ?? '';
    this._defaultCommandGroup = heading;
    return this;
  }
Set/get the default help group heading for subcommands added to this command.
(This does not override a group set directly on the subcommand using .helpGroup().)

@example
program.commandsGroup('Development Commands:);
program.command('watch')...
program.command('lint')...
...

@param {string} [heading]
@returns {Command | string}