input: Write a python function to insert an element at the beginning of a list.
  assert insert_at_beginning([2, 3, 4], 1) == [1, 2, 3, 4]
  assert insert_at_beginning(['b', 'c'], 'a') == ['a', 'b', 'c']
  assert insert_at_beginning([100, 200], 50) == [50, 100, 200]
output: def insert_at_beginning(lst, elem):
      return [elem] + lst