Use knip instead of unimported

[mroderick]

In https://github.com/sinonjs/sinon/pull/2636 we removed unimported, because it had been archived.

The author recommends using knip instead.

Let's implement knip and use it for finding unused files and dependencies.

[abidta]

I'd like to work on this, could you assign it to me?

[fatso83]

Sure, go ahead.