choices(values) {
    this.argChoices = values.slice();
    this.parseArg = (arg, previous) => {
      if (!this.argChoices.includes(arg)) {
        throw new InvalidArgumentError(
          `Allowed choices are ${this.argChoices.join(', ')}.`,
        );
      }
      if (this.variadic) {
        return this._concatValue(arg, previous);
      }
      return arg;
    };
    return this;
  }
Only allow argument value to be one of choices.

@param {string[]} values
@return {Argument}