preset(arg) {
    this.presetArg = arg;
    return this;
  }
Preset to use when option used without option-argument, especially optional but also boolean and negated.
The custom processing (parseArg) is called.

@example
new Option('--color').default('GREYSCALE').preset('RGB');
new Option('--donate [amount]').preset('20').argParser(parseFloat);

@param {*} arg
@return {Option}