input: Write a python function to download a webpage and save its content to a file.
  assert download_webpage("https://www.example.com/", "example.html") == None
  assert os.path.exists("example.html")
  with open("example.html", "r") as f:
      assert "<html>" in f.read()
output: import requests
  
  def download_webpage(url, filename):
      response = requests.get(url)
      with open(filename, 'w', encoding='utf-8') as file:
          file.write(response.text)