input: Write a python function to generate all 5-letter combinations from a given list of characters.
  assert generate_combinations(['a', 'e', 'i', 'o', 'u']) == ['aaaaa', 'aaaab', 'aaaac', ..., 'uuuui', 'uuuuj', 'uuuuk']
  assert generate_combinations(['x', 'y', 'z']) == ['xxxxx', 'xxxxy', 'xxxxyz', ..., 'zzzzz']
  assert generate_combinations(['1', '2']) == ['11111', '11112', '11121', '11122', ..., '22222']
output: def generate_combinations(chars):
      from itertools import product
      return [''.join(p) for p in product(chars, repeat=5)]