input: Write a function to extract verbs from a given sentence and return them as a list. Use the nltk library for part-of-speech tagging.
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_verbs("She loves to read books.")==['loves', 'read']
  assert extract_verbs("He went to the store.")==['went']
  assert extract_verbs("They are running fast.")==['are', 'running']
  assert extract_verbs("")==[]
output: def extract_verbs(sentence):
      from nltk import pos_tag, word_tokenize
      words = word_tokenize(sentence)
      tagged_words = pos_tag(words)
      verbs = [word for word, pos in tagged_words if pos.startswith('V')]
      return verbs