Rank index routes higher than layouts

[mjackson]

Index routes should rank higher than sibling layout routes with the same path.

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

    {/* When the URL is /layout, this route should match because it is more specific */}
    <Route path="layout" index element={...} />
  </Routes>
);

[mjackson]

This depends on #8144

[mjackson]

This was fixed in #8151