input: Write a python function to count letter frequency in a given text.
  assert count_letter_frequency("Hello, World!") == {'H': 1, 'e': 1, 'l': 3, 'o': 2, ',': 1, ' ': 1, 'W': 1, 'r': 1, 'd': 1, '!': 1}
  assert count_letter_frequency("Python") == {'P': 1, 'y': 1, 't': 1, 'h': 1, 'o': 1, 'n': 1}
  assert count_letter_frequency("Mississippi") == {'M': 1, 'i': 4, 's': 4, 'p': 2}
  assert count_letter_frequency("") == {}
output: def count_letter_frequency(text):
      frequency = {}
      for letter in text:
          if letter in frequency:
              frequency[letter] += 1
          else:
              frequency[letter] = 1
      return frequency