Allow wrapping raw synchronous databases as SqliteDatabase

[simolus3]

The asynchronous, isolate-based APIs provided by this package are a great default to access sqlite databases. For some test setups, they can stand in the way however. In particular,

1. They make it almost impossible to use in-memory databases (which are otherwise a great fit for tests).
2. The use of multiple isolates makes some async interactions much harder to debug (I ran into this writing tests for the PowerSync SDK, which was frustrating enough to make these changes here 😄 ).
3. Since the default factories set up workers, it's impossible to register custom functions in tests without compiling a custom worker for tests.

While it's a good thing that our unit tests have a setup very similar to what users have in practice, some individual components are easier to test when keeping everything on a single isolate. So, this:

1. Exports the existing SyncSqliteConnection as an easily-accessible factory wrapping a CommonDatabase.
2. Allows turning a single SqliteConnection into a SqliteDatabase.

There are documentation comments warning that these methods should not be used in apps, but they are helpful for tests.

[simolus3]

There's another issue I want to look at before a release, so I'll merge this without releasing.