`InitialNavigation` is used in `ExtraOptions`, which is already part of
the public API. Thus, `InitialNavigation` should be too. Not publicly
exporting it from `router/index.ts` seems an omission, since the type is
already annotated with the `@publicApi` JSDoc tag.
By publicly exporting `InitialNavigation`, it will also correctly appear
in the API docs on angular.io.
PR Close#32707