Export additional types

[ericallam]

We want to give users the ability to add custom transformers, and for that to work nicely with typescript we need these types. I tried to do export type but prettier threw an error (see prettier/prettier-vscode#2558), so I just exported them as values instead.

[Skn0tt]

These types are implementation details. Could you elaborate on your usecase? Maybe we can find a better solution using some TypeScript tricks.