input: Write a python function to split a given text into sentences using the NLTK library.
  assert split_into_sentences("Hello world. This is a test.") == ["Hello world.", "This is a test."]
  assert split_into_sentences("Can you believe it? Yes, I can!") == ["Can you believe it?", "Yes, I can!"]
  assert split_into_sentences("NLTK is a powerful tool. It helps with text processing.") == ["NLTK is a powerful tool.", "It helps with text processing."]
output: def split_into_sentences(text):
      import nltk
      nltk.download('punkt', quiet=True)
      from nltk.tokenize import sent_tokenize
      return sent_tokenize(text)