input: Write a python function to find the most common letter in a string and count its occurrences.
  assert findMostCommonLetter("abbabbccabbc") == 'b'
  assert countOccurences("abbabbccabbc", 'b') == 5
  assert findMostCommonLetter("abcdefgabcdefg") == 'a'
  assert countOccurences("abcdefgabcdefg", 'a') == 2
output: def findMostCommonLetter(s: str) -> str:
      from collections import Counter
      counter = Counter(s)
      most_common_letter, _ = counter.most_common(1)[0]
      return most_common_letter
  
  def countOccurences(s: str, letter: str) -> int:
      return s.count(letter)