Initial Drift QueryExecutor

[rkistner]

Tested and working:

• Querying
• Watching queries
• Insert, update and delete
• Custom queries
• Transactions, nested transactions

Upstream changes that could help to have in Drift:

1. A separate method for "update ... returning", instead of using runSelect for this.
2. Allow SupportedTransactionDelegate to implement nested transactions, instead of duplicating the logic here.

Structured as a monorepo to make it easy to merge into sqlite_async repo later.

This only contains basic examples showing the database setup. A full example of what usage would look like is here:
powersync-ja/powersync.dart#60. It's mostly just Drift examples though - the only code directly relevant to this package is the database setup.

[stevensJourney]

This is pretty awesome.