fix: fix usage of drizzle when core is used as a dep

[achou11]

Towards #278

Attempting to use @mapeo/core as a dep in a separate project breaks because:

1. We don't include the drizzle migration directories when publishing

2. The path string we provide to drizzle's migrate() function is not absolute, which breaks when this is being used as dependency (implementation of how Drizzle attempts to find the migration files based on the provided path is here)

I encountered these by using npm pack on this repo and then importing as a dep in separate project