code: function checkUrange(i) {
    const start = i;
    let l;
  
    if (i >= tokensLength) return 0;
  
    // Check for unicode prefix (u+ or U+)
    if (tokens[i].value === 'U' || tokens[i].value === 'u') i += 1;
    else return 0;
  
    if (i >= tokensLength) return 0;
  
    if (tokens[i].value === '+') i += 1;
    else return 0;
  
    while (i < tokensLength) {
      if (l = checkIdent(i)) i += l;
      else if (l = checkNumber(i)) i += l;
      else if (l = checkUnary(i)) i += l;
      else if (l = _checkUnicodeWildcard(i)) i += l;
      else break;
    }
  
    tokens[start].urangeEnd = i - 1;
  
    return i - start;
  }
docs: Check if token is a u-range (part of a unicode-range)
  (1) `U+416`
  (2) `U+400-4ff`
  (3) `U+4??`
  @param {number} i Token's index
  @return {number} Urange node's length