input: Write a python function to extract all list items text from a given HTML string using BeautifulSoup.
  assert extract_list_items('<div> <h1>This is an example block of HTML</h1> <ul> <li>This is the first item</li> <li>This is the second item</li> </ul> </div>') == ['This is the first item', 'This is the second item']
  assert extract_list_items('<ul><li>Item 1</li><li>Item 2</li><li>Item 3</li></ul>') == ['Item 1', 'Item 2', 'Item 3']
  assert extract_list_items('<div><ul><li>Single item</li></ul></div>') == ['Single item']
output: def extract_list_items(html_string):
      from bs4 import BeautifulSoup
      
      soup = BeautifulSoup(html_string, 'html.parser')
      list_items = soup.find_all('li')
      
      return [item.get_text() for item in list_items]