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

Create a new polyfill script at src/snek/static/polyfills/Promise.withResolvers.js that provides a fallback implementation of Promise.withResolvers for environments lacking native support. Include the polyfill as a module script in both app.html and base.html templates, ensuring it is loaded before app.js to guarantee availability at runtime.

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>
   -->