feat: Add type checking exercise

Move all packages except for the "production" one into dev dependencies, to emphasize that the stubs shouldn't even be available in production.

Exclude if TYPE_CHECKING: branches from coverage, since they are by definition never hit at runtime.

Code based on https://github.com/linz/geostore/blob/74e97730fbcb2b5eca1952732d2f71fcd059f19c/backend/import_dataset/task.py.