Make core fn Zod import type-only (#27103)

packages/core/src/util/fn.ts

@@ -1,4 +1,4 @@
-import { z } from "zod"
+import type { z } from "zod"
 
 export function fn<T extends z.ZodType, Result>(schema: T, cb: (input: z.infer<T>) => Result) {
   const result = (input: z.infer<T>) => {