input: Write a python function to add a given number to each element in a list.
  assert add_number([1, 2, 3], 5) == [6, 7, 8]
  assert add_number([10, 20, 30], 10) == [20, 30, 40]
  assert add_number([0, -1, -2], 2) == [2, 1, 0]
output: def add_number(lst, num):
      return [x + num for x in lst]