Expose generic type which extracts type of a store's underlying value.

[joeally]

Describe the problem

In the same way that typescript offers Awaited to extract the type from a promise it would be nice if we could provide an easy way to get the type of the underlying value of a store.

The reason I came up with this is because I wanted to extract to type from the page store in sveltekit like below:

import type { page } from '$app/stores';

type PageType = StoreValue<typeof page>;

function acceptsPageStoreValue(value: PageType): ArbitraryType {
    ...
}

Of course I could manually copy the actual type which is:

Page<Record<string, string>, string | null>

But this is much less readable.

Describe the proposed solution

Add the below type to runtime/store/index.ts

  type StoreValue<T extends Readable<unknown>> = Parameters<Parameters<T['subscribe']>[0]>[0];

I'm not sure if StoreValue is a good name here. But if we decide on a good name I'd be happy to put in a PR. Especially as it's such a simple change.

Alternatives considered

The alternative is not simply to not bother with this. Devs can trivially implement this type themselves if they want.

Importance

nice to have