Allow path on index routes

[mjackson]

We should allow index routes to have a path. This eliminates the need to nest them inside their parent route.

e.g. this should be a totally valid route config:

let routes = (
  <Routes>
    <Route path="layout" element={...}>
      <Route path="page" element={...} />
    </Route>
    <Route path="layout" index element={...} />
  </Routes>
);

Implementation

I don't think any implementation actually needs to change here, just the IndexRouteProps declaration. But we should write a test or two just to be sure.

[mjackson]

Fixed in #8148