longestOptionTermLength(cmd, helper) {
    return helper.visibleOptions(cmd).reduce((max, option) => {
      return Math.max(
        max,
        this.displayWidth(helper.styleOptionTerm(helper.optionTerm(option))),
      );
    }, 0);
  }
Get the longest option term length.

@param {Command} cmd
@param {Help} helper
@returns {number}