input: Write a python function to extract all the words starting with a specific letter.
  assert extract_words('This is a sample sentence to test the program', 's') == ['sample', 'sentence']
  assert extract_words('She sells sea shells by the sea shore', 's') == ['She', 'sells', 'sea', 'shells', 'sea', 'shore']
  assert extract_words('Python programming language', 'p') == ['programming']
output: def extract_words(text, letter):
      words = text.split()
      return [word for word in words if word.lower().startswith(letter.lower())]