Export WorkerPool related types to make pool typing easier

Hello. First of all, thank you for this great library.

Would it be possible to export `ExposedToThreadType`, `QueuedTask` and `WorkerPool` types to use in our apps? With these types available it would be much easier to type Pool and/or array of tasks inside our apps like:

```ts
import { Pool, spawn, ExposedToThreadType, QueuedTask, WorkerPoolType } from 'threads';

...

class App {
queue: QueuedTask, boolean>[];
pool: WorkerPool>;
...
```

or is there any other way to type these variables? Thank you!