Re: [w3c/ServiceWorker] Top-level await integration for ServiceWorkers running modules (#1407)

With [this pull](https://github.com/w3c/ServiceWorker/issues/1356) being merged, currently there is no way to import a module that uses TLA whatsoever.

I understand the reason is that `import()` invokes fetch and so doesn't work offline, but could we get some way to prefetch/parse modules so that the service worker can still asynchronously evaluate them? Which is to say, we declare them upfront synchronously and only those modules can be dynamically imported later:

```js
// Service worker can treat ./moduleWithTLA.js and it's subgraph as necessary for 
// the service worker, so it should fetch this graph as well, the service worker would
// only become active once the graph for ./moduleWithTLA.js has successfully fetched and
// parsed, it does not however block *evaluation* of the current module
registerModule("./moduleWithTLA.js");

self.addEventListener("some-event", async (evt) => {
  evt.waitUntil(async () => {
    // Allowed, triggers no fetch as once this service worker is active, ./moduleWithTLA.js
    // is already saved and cached with the rest of the modules
    await import("./moduleWithTLA.js");
    
    // Not allowed, this module wasn't registered, so it wasn't fetched before activating
    // the service worker, as such `import()` immediately throws an error
    await import("./notRegistered.js");
  });
});
``` 

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