[feat] More Flutter Helpers

[dickermoshe]

• A single function for opening the database
• ... More as we think of what FlutterDevs could use

@simolus3 What are your thoughts on adding some Flutter specific code to this package?

[simolus3]

Back when drift was called moor, there was a moor_flutter package. It only contained a sqflite-based executor that would now live in drift_sqflite, but I agree that having a drift_flutter package which contains the _openConnection() method as a public API (so that we don't have to suggest that monstrous snippet on the getting started docs) is definitely nice to have.
Core package:drift can't depend on Flutter which is the reason for the current split, but having a dedicated Flutter package implementing best practices is a good call.