[router] fix relative navigation on hoisted routes

[marklawlor]

Why

Fix #27753

How

This isIndexPath function was not taking in account hoisted index routes that may not have the param structure

Take the following structure.

_layout // Stack
parent/
└── child/
    ├── index // name: parent/child/index
    └── page  // name: parent/child/page 

Because there are no route params or nested _layout, all routes are hoisted to the root _layout. The existing logic for only checking for route.params is insufficient as it will not exist. Additionally we cannot check for name: 'index' as hoisted routes have complex names.

Test Plan

I think our existing tests are fine, just added one for this scenario

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).

[EvanBacon]

nice work