vendors/tempy.js/fillLetters
    var fillLetters = (start, end, step = 1, options = {}) => {
      if (!isNumber(start) && start.length > 1 || !isNumber(end) && end.length > 1) {
        return invalidRange(start, end, options);
      }
      let format = options.transform || ((val) => String.fromCharCode(val));
      let a = `${start}`.charCodeAt(0);
      let b = `${end}`.charCodeAt(0);
      let descending = a > b;
      let min = Math.min(a, b);
      let max = Math.max(a, b);
      if (options.toRegex && step === 1) {
        return toRange(min, max, false, options);
      }
      let range = [];
      let index = 0;
      while (descending ? a >= b : a <= b) {
        range.push(format(a, index));
        a = descending ? a - step : a + step;
        index++;
      }
      if (options.toRegex === true) {
        return toRegex(range, null, { wrap: false, options });
      }
      return range;
    };