Support code splitting in web workers

[devongovett]

We currently don't support code splitting in web workers via import(), because the bundle loader for JS relies on the DOM. We should extend this to also support web workers.

This will allow moving common dependencies between workers, or even between workers and the main process to a separate bundle so it only gets loaded once. See https://twitter.com/DasSurma/status/1013489346090012672. cc. @surma.

In order to be consistent about how require() works, we won't make it automatically create a split point when called in a worker, as that would cause the code to potentially be removed from another bundle. But if you use import() to load the shared module in both the main bundle and a worker, the module would be placed into its own bundle and loaded in both places.

Related: #758

[abaybektursun]

What's the workaround?

[devongovett]

I believe this is fully supported as of #4309!