Improvements to shortcuts computations : consider upstream and downstream nodes

I'm creating an issue to track a old comment made here : https://github.com/a-b-street/ltn/issues/69#issuecomment-2600882440


https://www.openstreetmap.org/#map=17/43.600230/3.902979

Both of those shortcuts aren't *shortcuts* because you could reach the outcome faster by another path when entering the shortcut.

> We could check what's allowed at the exit intersection. From that exit, if the only reachable point is the entry intersection, then it's this simple redundant case. If there's anything in between, then maybe it's still a shortcut somebody would take, but it could be weighted less, based on how "useful" exiting there is.

So I would suggest that you should not only take the first-downstream node of the exit, you should also take the first upstream node of the entry.

Most of the shortcuts shown here would be discarded or changed


For example, this shortcut would never be taken as a car would come from upstream(A) which would give another shortcut