input: Write a function that extracts all the words starting with a specific letter from a given string. The function should take two parameters: the string and the specific letter, and return a list of words that start with that letter.
  assert extract_words('This is a sample sentence to test the program', 's') == ['sample', 'sentence', 'to', 'test']
  assert extract_words('The quick brown fox jumps over the lazy dog', 't') == ['The', 'the']
  assert extract_words('Hello world', 'a') == []
  assert extract_words('', 's') == []
  assert extract_words('Python is fun', 'P') == ['Python']
output: def extract_words(s, letter):
      return [word for word in s.split() if word.lower().startswith(letter.lower())]