fix(router): support push going back to sibling with nested stack from a modal.

[EvanBacon]

Why

• Fix issue reported by @blainekasten.
• Increase logic in push to account for moving between sibling routes which all use the leaf-most name index.

How

• If the target route name is index, then use the parent state as the route target. React Navigation would search for index when moving to slot/index via /slot. This would result in the wrong route being selected:

"routeNames": [
  "index",
  "slot",
  "(group)",
  "_sitemap",
  "[...404]"
],

Now we account for index routes by pushing up to slot so the StackRouter can identify the right screen.

Test Plan

• The issue could be reproduced using our testing library.