vendors/execa.js/step
    var which = (cmd, opt, cb) => {
      if (typeof opt === "function") {
        cb = opt;
        opt = {};
      }
      if (!opt)
        opt = {};
      const { pathEnv, pathExt, pathExtExe } = getPathInfo(cmd, opt);
      const found = [];
      const step = (i) => new Promise((resolve, reject) => {
        if (i === pathEnv.length)
          return opt.all && found.length ? resolve(found) : reject(getNotFoundError(cmd));
        const ppRaw = pathEnv[i];
        const pathPart = /^".*"$/.test(ppRaw) ? ppRaw.slice(1, -1) : ppRaw;
        const pCmd = path3.join(pathPart, cmd);
        const p = !pathPart && /^\.[\\\/]/.test(cmd) ? cmd.slice(0, 2) + pCmd : pCmd;
        resolve(subStep(p, i, 0));
      });
      const subStep = (p, i, ii) => new Promise((resolve, reject) => {
        if (ii === pathExt.length)
          return resolve(step(i + 1));
        const ext = pathExt[ii];
        isexe(p + ext, { pathExt: pathExtExe }, (er, is) => {
          if (!er && is) {
            if (opt.all)
              found.push(p + ext);
            else
              return resolve(p + ext);
          }
          return resolve(subStep(p, i, ii + 1));
        });
      });
      return cb ? step(0).then((res) => cb(null, res), cb) : step(0);
    };