executableDir(path) {
    if (path === undefined) return this._executableDir;
    this._executableDir = path;
    return this;
  }
Get or set the directory for searching for executable subcommands of this command.

@example
program.executableDir(__dirname);
// or
program.executableDir('subcommands');

@param {string} [path]
@return {(string|null|Command)}