Expose types so typescript users can use them

https://github.com/andywer/threads.js/blob/762337476637f83f38876b9f25ec5e5816c47594/src/master/spawn.ts#L23-L25

I need to write this:

```ts
private wikiWorkers: Record = {};
```

but `Module '"threads"' has no exported member 'ArbitraryThreadType'.ts(2305)`