scriptURL of serviceWorker.register isn't considered part of bundle

👋🏼

I'm currently working with microbundle to package a service worker library. I have a file [`index.mjs`](https://github.com/TimDaub/web3-service-worker-cache/blob/demo/src/index.mjs) that takes care of initializing and handling the SW in the main thread. And I have a file [`worker.mjs`](https://github.com/TimDaub/web3-service-worker-cache/blob/demo/src/worker.mjs) that separates the service worker code.

After browser compatability, I register the worker like this:

```js
await navigator.serviceWorker.register('./worker.mjs');
```

I use `microbundle` in a standard way by configuring it in [package.json](https://github.com/TimDaub/web3-service-worker-cache/blob/demo/package.json).

I'm aware that no bundler would take the input of `register(scriptURI)` and make it somehow part of the output. Usually, a developer has to care for this separately e.g. in rollup by adding a secondary input and output. But today while doing that I found it annoying and I asked myself why?

Couldn't microbundle just take care of transpiling the worker.mjs file and include it somehow? E.g. it could be bundled as a base64 data URI...

Review the package.json configuration and the index.mjs and worker.mjs entry files linked in the issue, then inspect how microbundle handles additional Rollup inputs. Determine the expected behavior for a serviceWorker.register('./worker.mjs') reference, including whether transpiling or embedding it is feasible. Done should be a decided, documented implementation scope with a reproducible example or test.

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