input: Write a python function to compare the length of two strings and return the longer one.
  assert compare_string_length("apple", "banana") == "banana"
  assert compare_string_length("cat", "elephant") == "elephant"
  assert compare_string_length("dog", "fox") == "dog"
output: def compare_string_length(str1, str2):
      if len(str1) > len(str2):
          return str1
      else:
          return str2