fix: update onMount type to allow async to return any (#8714)

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

packages/svelte/src/runtime/internal/lifecycle.js

@@ -36,9 +36,7 @@ export function beforeUpdate(fn) {
  *
  * https://svelte.dev/docs#run-time-svelte-onmount
  * @template T
- * @param {() => T extends Promise<() => any>
- * 		? "Returning a function asynchronously from onMount won't call that function on destroy"
- * 		: T} fn
+ * @param {() => import('./private.js').NotFunction<T> | Promise<import('./private.js').NotFunction<T>> | (() => any)} fn
  * @returns {void}
  */
 export function onMount(fn) {

packages/svelte/src/runtime/internal/private.d.ts

@@ -123,3 +123,8 @@ export interface Task {
 	abort(): void;
 	promise: Promise<void>;
 }
+
+/**
+ * Anything except a function
+ */
+type NotFunction<T> = T extends Function ? never : T;

packages/svelte/test/types/on-mount.ts

@@ -51,8 +51,15 @@ onMount(async () => {
 	};
 });
 
-// @ts-expect-error async and return any
+// async and return any
 onMount(async () => {
 	const a: any = null as any;
 	return a;
 });
+
+// async and return function casted to any
+// can't really catch this without also catching above
+onMount(async () => {
+	const a: any = (() => {}) as any;
+	return a;
+});