[2.x] Update NavItem class to support deferring route resolves

[emmadesilva]

Part of #1538 and allows for deferring the route link evaluation instead of forcing it at construction time. This has the side effect that the destination method now may return a route object instead of string. This targets v2.

[what-the-diff]

PR Summary

• Improved URL Retrieval Process
  We've added a new method into the NavItem class for retrieving the link URL. This was previously embedded in the NavItem::getDestination() logic and is now independently handled by NavItem::getLink(), leading to improved code readability and organization.

• Enhanced Routing in NavItem
  The NavItem::getDestination() method has been modified to return the Route instance if it is the destination type. This provides more flexibility when managing routes.