code: function checkParentSelectorExtension(i) {
    const start = i;
    let l;
  
    if (i >= tokensLength) return 0;
  
    while (i < tokensLength) {
      if (l = checkIdentOrInterpolation(i) || checkPartialIdent(i)) i += l;
      else break;
    }
  
    return i - start;
  }
docs: Check if token is a parent selector extension, e.g. `&--foo-bar`
  @param {number} i Token's index number
  @returns {number} Length of the parent selector extension