input: Write a python function to count the frequency of each letter in a given text and return a list of lists with each letter and its frequency.
  assert letter_frequency("hello world") == [['h', 1], ['e', 1], ['l', 3], ['o', 2], ['w', 1], ['r', 1], ['d', 1]]
  assert letter_frequency("test") == [['t', 2], ['e', 1], ['s', 1]]
  assert letter_frequency("aabbcc") == [['a', 2], ['b', 2], ['c', 2]]
output: def letter_frequency(text):
      frequency = {}
      for letter in text:
          if letter.isalpha():  # Consider only alphabetic characters
              if letter in frequency:
                  frequency[letter] += 1
              else:
                  frequency[letter] = 1
      return [[letter, count] for letter, count in sorted(frequency.items())]