input: Write a python function to find the length of the longest string in a list.
  assert longest_string_length(["I", "am", "so", "happy"]) == 5
  assert longest_string_length(["short", "longer", "longest", "tiny"]) == 7
  assert longest_string_length(["apple", "banana", "cherry", "date"]) == 6
output: def longest_string_length(lst):
      return max(len(s) for s in lst)