input: Write a python function to return the frequency of each letter in a string.
  assert characterFrequency("hello world") == {'l': 3, 'o': 2, 'h': 1, 'e': 1, 'd': 1, 'r': 1, 'w': 1}
  assert characterFrequency("test") == {'t': 2, 'e': 1, 's': 1}
  assert characterFrequency("character") == {'c': 2, 'h': 2, 'a': 2, 'r': 2, 't': 1, 'e': 1}
output: def characterFrequency(s):
      frequency = {}
      for char in s:
          if char.isalpha():  # Ensure we only count letters, ignoring spaces and other characters
              frequency[char] = frequency.get(char, 0) + 1
      return frequency