longestSubcommandTermLength(cmd, helper) {
    return helper.visibleCommands(cmd).reduce((max, command) => {
      return Math.max(
        max,
        this.displayWidth(
          helper.styleSubcommandTerm(helper.subcommandTerm(command)),
        ),
      );
    }, 0);
  }
Get the longest command term length.

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