[fix] more specific return type for crossfade

People with "noUncheckedIndexedAccess" set to true in their tsconfig would get false errors when accessing the return type because without this spefici tuple typing, TS infers this as being an array of functions, not a tuple of functions.
Fixes #6926

src/runtime/transition/index.ts

@@ -212,7 +212,20 @@ type ClientRectMap = Map<any, { rect: ClientRect }>;
 
 export function crossfade({ fallback, ...defaults }: CrossfadeParams & {
 	fallback?: (node: Element, params: CrossfadeParams, intro: boolean) => TransitionConfig;
-}) {
+}): [
+  (
+    node: Element,
+    params: CrossfadeParams & {
+      key: any;
+    }
+  ) => () => TransitionConfig,
+  (
+    node: Element,
+    params: CrossfadeParams & {
+      key: any;
+    }
+  ) => () => TransitionConfig
+] {
 	const to_receive: ClientRectMap = new Map();
 	const to_send: ClientRectMap = new Map();