fix: correctly infer the type of the function passed to derived

Before this commit, the type of the function passed to derived was not
correctly inferred by typescript in the async case. By putting the async
case first in the list of overloads of the derived function, it looks like
it fixes the issue, and the type is still correctly inferred in the sync
case too.

src/index.spec.ts

@@ -971,6 +971,26 @@ describe('stores', () => {
       });
     });
 
+    it('should infer types automatically in the async case', () => {
+      const a = writable(1);
+      const b = writable(2);
+      const sum = derived(
+        [a, b],
+        ([a, b], set) => {
+          set(a + b);
+        },
+        0
+      );
+      expect(get(sum)).toBe(3);
+    });
+
+    it('should infer types automatically in the sync case', () => {
+      const a = writable(1);
+      const b = writable(2);
+      const sum = derived([a, b], ([a, b]) => a + b);
+      expect(get(sum)).toBe(3);
+    });
+
     it('should call clean-up function returned in deriveFn with derived', () => {
       const a = writable(1);
       const cleanUpFn = jasmine.createSpy('cleanupFn').and.callThrough();

src/index.ts

@@ -752,13 +752,13 @@ export abstract class DerivedStore<
  */
 export function derived<T, S extends SubscribableStores>(
   stores: S,
-  options: SyncDeriveFn<T, S> | SyncDeriveOptions<T, S>,
-  initialValue?: T
+  options: AsyncDeriveFn<T, S> | AsyncDeriveOptions<T, S>,
+  initialValue: T
 ): Readable<T>;
 export function derived<T, S extends SubscribableStores>(
   stores: S,
-  options: AsyncDeriveFn<T, S> | AsyncDeriveOptions<T, S>,
-  initialValue: T
+  options: SyncDeriveFn<T, S> | SyncDeriveOptions<T, S>,
+  initialValue?: T
 ): Readable<T>;
 export function derived<T, S extends SubscribableStores>(
   stores: S,