Move declaration merging to routerTree.gen.ts

[kolodny]

It's possible to append these lines to routeTree.gen.ts

const router = false ? createRouter({ routeTree }) : null!;

declare module '@tanstack/react-router' {
  interface Register {
    router: typeof router;
  }
}

Which allows us to not need to bother the user about registering the types in main.tsx. This feel like a better OOTB experience.