code: function checkUri(i) {
    const start = i;
  
    if (i >= tokensLength || tokens[i++].value !== 'url' ||
        i >= tokensLength || tokens[i].type !== TokenType.LeftParenthesis)
      return 0;
  
    return tokens[i].right - start + 1;
  }
docs: Check if token is part of URI (e.g. `url('/css/styles.css')`)
  @param {Number} i Token's index number
  @returns {Number} Length of URI