code: function checkPercentage(i) {
    const start = i;
    let l;
  
    if (i >= tokensLength) return 0;
  
    if (l = checkNumber(i)) i += l;
    else return 0;
  
    if (i >= tokensLength) return 0;
  
    // Skip `%`.
    if (tokens[i].type === TokenType.PercentSign) i++;
    else return 0;
  
    return i - start;
  }
docs: Check if token is part of a number with percent sign (e.g. `10%`)
  @param {Number} i Token's index number
  @returns {Number}