input: Write a function to take in two strings as input and return a list of all the words in both strings. The function should split each string into words and then concatenate the two lists of words.
  assert string_to_list("I want to make a difference", "The journey to success is long")==['I', 'want', 'to', 'make', 'a', 'difference', 'The', 'journey', 'to', 'success', 'is', 'long']
  assert string_to_list("Hello world", "world of programming")==['Hello', 'world', 'world', 'of', 'programming']
  assert string_to_list("Python", "is fun")==['Python', 'is', 'fun']
  assert string_to_list("", "testing empty string")==['testing', 'empty', 'string']
  assert string_to_list("Only one string", "")==['Only', 'one', 'string']
output: def string_to_list(str1, str2):
      return str1.split() + str2.split()