Graphile worker should export LogLevel as a type

[Zehelein]

The worker exposes the 'Logger' type but not the 'LogLevel': https://github.com/graphile/worker/blob/main/src/index.ts

When creating a custom logFactory we would need that LogLevel to call our own implementation based on the LogLevel.

Graphile migrate for example does export it: https://github.com/graphile/migrate/blob/main/src/index.ts