Add in-memory database for tests

[simolus3]

This adds the PowerSyncDatabase.inMemory factory to create PowerSync databases backed by a single in-memory connection.

This utility simplifies the test setup for users:

1. It doesn't need an open factory, so users can write tests in commonIntegrationTest instead of having to setup expect / actuals in tests.
2. One doesn't have to create a temporary file path for each test, databases are isolated on their own.
3. No cleanup logic is required.

We should continue testing the core logic with "real" file-backed databases to ensure the pool works, but having a single in-memory connection is useful to test other packages that can expect :core to behave correctly. This adopts that for the SQLDelight integration, I'm also writing tests for the Supabase connector where these changes would simplify the setup.