feat: add typings to package

[unional]

Hi, @bearjaws ,

Thanks for making this package.

This PR adds TypeScript types to the package.
Please see if you would like to add it in.

I have made a slight adjustment in the test so that it can be used to check the types.
It will be beneficial down the road to help to detect any inconsistency between the code and the types.

All it does is change the reference point from ../lib/wget to .. to reference the package itself, and add the @ts-check comment at the top.

If you want, I can remove that change too.

Best

[bearjaws]

Thanks for this, I love typescript and think this will help others greatly.

👍