input: Write a python function to normalize a list of strings by stripping whitespace and converting to lowercase.
  assert normalize_strings([" HELLO  ", " WorLD  ", "HeLlo", "DELL"]) == ["hello", "world", "hello", "dell"]
  assert normalize_strings(["  Python  ", "  ProGramming  ", "  Fun  "]) == ["python", "programming", "fun"]
  assert normalize_strings([" TeST ", " sTrING ", "  VALUE "]) == ["test", "string", "value"]
output: def normalize_strings(strings):
      return [s.strip().lower() for s in strings]