code: public function sortKeysBy(callable $iteratee, $order = SORT_ASC)
      {
          return $this->sortKeysWith($this->iterateeToComparator($iteratee, $order === SORT_ASC));
      }
docs: Returns a bag with the keys sorted based on the `$iteratee` function.
  
  Example:
  
  $bag = Bag::from([
  'blue'  => 'a',
  'red'   => 'b',
  'black' => 'c',
  ]);
  
  // Sort keys by first letter
  $bag->sortKeysBy(function ($key) {
  return $key[0];
  });
  // Bag of ['blue' => 'a', 'black' => 'c', 'red' => 'b']
  
  @param callable $iteratee Function given `($key)`
  @param int      $order    `SORT_ASC` or `SORT_DESC`
  
  @return static