Before #41162, angular.io was broken on IE 11 due to missing a polyfill
for an API (`Reflect.construct()`) needed by the Custom Elements ES5
shim. #41162 tried to fix this by loading the necessary polyfill
(`es.reflect.construct.js`) on browsers that do not support ES2015
modules (including IE 11).
It turns out that the fix in #41162 was itself broken, because the
`es.reflect.consruct.js` script (included directly in the page via a
`<script>` tag) was in CommonJS format (which cannot run in the browser
as is). By chance, this still allowed browsers that supported neither
Custom Elements nor ES2015 modules (such as IE 11) to work correctly as
a side-effect of loading the `@webcomponents/custom-elements` polyfill
after the Custom Elements ES5 shim (`native-shim.js`). However, on the
few browsers that natively support Custom Elements but not ES2015
modules, angular.io would still be broken.
This commit correctly fixes angular.io on all browsers by properly
bundling the polyfills and transpiling to ES5.
Implementation-wise, we use [esbuild][1] for bundling the polyfills (and
converting from CommonJS to a browser-compatible, IIFE-based format) and
[swc][2] for downleveling the code to ES5 (since `esbuild` only supports
ES2015+).
[1]: https://esbuild.github.io/
[2]: https://swc.rs/
PR Close#41183
George Kalpakas