Proposal: Merge web workers

[simolus3]

The web SDK currently uses two web workers: A shared worker driving the sync process across tabs, and a dedicated worker providing asynchronous database access.

In our Dart SDK, merging workers resulted in a 28% decrease of total worker size due to overlapping code being bundled into both workers. It's unlikely we'll see the same numbers here since our sync worker doesn't bundle wa-sqlite which makes up the bulk of the size, but it's probably still something worth doing to simplify the setup.

This is a breaking change for users with a custom worker setup, but we would continue exposing internals to help you write your own workers. The worker would detect whether it's running in a dedicated or shared scope to act as a database or sync worker, respectively.