Add missing Drift import for electric types

[etienneolivier]

Hello and thank you very much for this project! 🙏

Just a small proposal to complete the documentation on integrating Electric into an existing Flutter application.

I had a compilation error for the missing Electric types in the database file generated by Drift without this import.

Have a nice day!

[davidmartos96]

@etienneolivier Thank you! You are right, otherwise Drift won't find the type conversions.