Improve TypeScript type definition for stores

[albertms10]

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.

[benmccann]

This is defined here:

sapper/runtime/index.d.ts

Line 15 in f5032a5

export const stores: () => unknown;

[TheComputerM]

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

[albertms10]

Of course!