input: Write a python function to sort characters in a string by their frequency in descending order.
  assert sort_char_occurrences("banana") == ['a', 'n', 'b']
  assert sort_char_occurrences("apple") == ['p', 'a', 'l', 'e']
  assert sort_char_occurrences("mississippi") == ['i', 's', 'p', 'm']
output: def sort_char_occurrences(s):
      from collections import Counter
      return [char for char, _ in Counter(s).most_common()]