`sqlite_utils.utils.TypeTracker` should be a documented API

I've used it in a couple of external places now:

- https://github.com/simonw/datasette-socrata/blob/32fb256a461bf0e790eca10bdc7dd9d96c20f7c4/datasette_socrata/__init__.py#L264-L280
- https://github.com/simonw/datasette-lite/blob/caa8eade10f0321c64f9f65c4561186f02d57c5b/webworker.js#L55-L64

Refs:
- https://github.com/simonw/datasette-lite/issues/32