Switch to polyfill cloudflare cdn (#197)

There has been a polyfill.io cdn supply chain attack. It is advised to remove all references to this cdn immediately.

README.md

@@ -75,7 +75,7 @@ to download only the polyfills needed by the current browser. Just add the
 following line to the start of your page:
 
 ```html
-<script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=Map,Set,Element.prototype.matches,Node.prototype.contains"></script>
+<script src="https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?version=4.8.0&features=Map%2CSet%2CElement.prototype.matches%2CNode.prototype.contains"></script>
 ```
 
 ### Strict Content Security Policy

demo/index.html

@@ -132,7 +132,7 @@ <h3>This inner section is <span class="inner-not">not</span> marked inert</h3>
       </div>
     </section>
 
-    <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=Map,Set,Element.prototype.matches,Node.prototype.contains"></script>
+    <script src="https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?version=4.8.0&features=Map%2CSet%2CElement.prototype.matches%2CNode.prototype.contains"></script>
     <script src="https://unpkg.com/wicg-inert"></script>
     <script>
       var toggler = document.querySelector('#toggle');