Update Types to Match React Router Updates

Fixes a type mismatch caused by change remix-run/react-router#9366 in react-router code.

Author: philipatkinson

packages/react/src/reactrouterv6.tsx

@@ -8,14 +8,24 @@ import React from 'react';
 
 import { Action, Location } from './types';
 
-interface RouteObject {
-  caseSensitive?: boolean;
-  children?: RouteObject[];
-  element?: React.ReactNode;
-  index?: boolean;
-  path?: string;
+interface NonIndexRouteObject {
+    caseSensitive?: boolean;
+    children?: RouteObject[];
+    element?: React.ReactNode | null;
+    index?: false;
+    path?: string;
 }
 
+interface IndexRouteObject {
+    caseSensitive?: boolean;
+    children?: undefined;
+    element?: React.ReactNode | null;
+    index?: true;
+    path?: string;
+}
+
+type RouteObject = IndexRouteObject | NonIndexRouteObject;
+
 type Params<Key extends string = string> = {
   readonly [key in Key]: string | undefined;
 };