feat: add Promise.withResolvers polyfill and include it in app and base templates

The polyfill provides a static method on Promise that returns an object with resolve and reject functions exposed, enabling external resolution of a promise. It is loaded as a module script in both app.html and base.html to ensure compatibility before app.js runs.

src/snek/templates/app.html

@@ -6,6 +6,7 @@
     <link rel="manifest" href="/manifest.json" />
   <title>Snek</title>
   <style>{{highlight_styles}}</style>
+  <script src="/polyfills/Promise.withResolvers.js" type="module"></script>
   <!-- 
   <script src="/push.js"></script>
   -->