fix: align error typings with react, from unknown to error/null

See
```ts
if (error) return 'An error has occurred: ' + error.message
```

in react/simple example

Author: JulianCataldo

packages/lit-query/src/QueryController.ts

@@ -21,7 +21,7 @@ export type { QueryObserverOptions }
  */
 export class QueryController<
   TQueryFnData = unknown,
-  TError = unknown,
+  TError = Error | null,
   TData = TQueryFnData,
   TQueryData = TQueryFnData,
   TQueryKey extends QueryKey = QueryKey,