fix(start-client-core): export OptionalFetcher type (#4001)

Co-authored-by: Sean Cassiere <33615041+SeanCassiere@users.noreply.github.com>

Author: dccarmo

packages/start-client-core/src/index.tsx

@@ -75,6 +75,7 @@ export type {
   NextFn,
   Method,
   StaticCachedResult,
+  OptionalFetcher,
 } from './createServerFn'
 export {
   applyMiddleware,