input: Write a python function to crawl a website and print all unique URLs found on the webpage.
  assert webcrawler('https://www.example.com') is None
  assert webcrawler('https://www.test.com') is None
output: def webcrawler(start_url):
      import requests
      from bs4 import BeautifulSoup
      from urllib.parse import urljoin
      import re
      
      visited = set()
      def crawl(url):
          if url in visited:
              return
          visited.add(url)
          try:
              response = requests.get(url)
              if response.status_code != 200:
                  return
              soup = BeautifulSoup(response.text, 'html.parser')
              for link in soup.find_all('a', href=True):
                  href = urljoin(url, link['href'])
                  if re.match(r'^https?://', href):
                      crawl(href)
          except requests.RequestException:
              return
      
      crawl(start_url)
      for url in visited:
          print(url)