Improve TypeScript type definition for stores

Is your feature request related to a problem? Please describe.
The stores function from @sapper/app has an unknown return type, although there is a known object returning some predefined values such as preloading, page, session, as seen in the Sapper Docs.

<script>
  import { stores } from '@sapper/app';
  const { preloading, page, session } = stores(); // => `unknown`, destructured items are `any`

  const delayedPreloading = derived(
    preloading,
    (currentPreloading, set) => { // as preloading is treated as `any`, TypeScript cannot infer types for fn parameters
      // ...
    }
  );

  // ...
</script>

Describe the solution you'd like
Adding types for these stores would improve DX.

Describe alternatives you've considered
Just ignore TypeScript warning about the any type in use cases.

This is defined here:

sapper/runtime/index.d.ts

Line 15 in f5032a5

export const stores: () => unknown;

Can I try to fix this issue and open a PR?

Of course!