RFC: block or confirm route transitions

## Source
- atomic-router/atomic-router#28

## Summary
Built-in way to prevent or confirm user transitions from one route to another in effector/router.

## Motivation
Some apps need to stop navigation unless a condition is met (for example, a dirty form) or show a confirmation flow before leaving a route.

## Proposal
- Expose a route transition guard / blocker API for route composition.
- Allow a store/event-driven confirmation flow to decide whether to proceed.
- Optionally support allow/prevent lists for destination routes.

## Notes
This is adapted from the atomic-router request into effector/router terminology. I searched effector/router for blocker, confirm, beforeunload, beforeOpen, transition, and navigation duplicates and did not find an existing issue for this request.

## Relates
- #42