[fix] relax store derived function signature (#6509)

With this fix, doing
```
const stores= [writable(0), writable(1)]
derived(stores, ([a,b,c]) => {});
```
works without type errors. The propblem with the previous type signature was that it's too strict for TypeScript's suboptimal inference of the value `stores` in that example, which is `Array<Readable<any>>`, which does not convey the info "at least one element", which the old Stores signature required.
Runtime-wise, it's no problem passing an empty array to derived.

The new signature is only appended to, not replaced, because the old signature is able to correctly infer the values of each array entry: For `derived([writable(0), writable('foo')], ([a, b]) => {});` the parameters `a` and `b` are correctly inferred as `number` and `string`. If the type would be changed to `type Stores = Readable<any> | Array<Readable<any>>` that would be no longer the case.

Fixes #6178

CHANGELOG.md

@@ -16,6 +16,7 @@
 * Add a11y warning `a11y-mouse-events-have-key-events` which checks that `mouseover`/`mouseout` are accompanied by `focus`/`blur` event handlers ([#5938](https://github.com/sveltejs/svelte/pull/5938))
 * Remove deprecated a11y warning `a11y-no-onchange warning` ([#6457](https://github.com/sveltejs/svelte/issues/6457))
 * Stop checking `a11y-media-has-caption` a11y warning on `<audio>` elements ([#6054](https://github.com/sveltejs/svelte/issues/6054))
+* Relax `derived` function signature ([#6178](https://github.com/sveltejs/svelte/issues/6178))
 
 ## 3.38.3
 

src/runtime/store/index.ts

@@ -113,7 +113,7 @@ export function writable<T>(value?: T, start: StartStopNotifier<T> = noop): Writ
 }
 
 /** One or more `Readable`s. */
-type Stores = Readable<any> | [Readable<any>, ...Array<Readable<any>>];
+type Stores = Readable<any> | [Readable<any>, ...Array<Readable<any>>] | Array<Readable<any>>;
 
 /** One or more values from `Readable` stores. */
 type StoresValues<T> = T extends Readable<infer U> ? U :