input: Write a Python function that generates a list of all possible prefixes of a given string, ordered by increasing length.
  assert list_prefixes('') == []
  assert list_prefixes('asdfgh') == ['a', 'as', 'asd', 'asdf', 'asdfg', 'asdfgh']
  assert list_prefixes('WWW') == ['W', 'WW', 'WWW']
output: def list_prefixes(s):
      return [s[:i] for i in range(1, len(s) + 1)]