Improvements to Design LTN : Improved shortcut detection

## Current behaviour:

When enumerating shortcuts in the LTN tool, it is possible to iterate through all of the potential shortcuts for a specific road segment.

The shortcut detection will respect will turn restrictions which are *within* a neighbourhood.

The shortcut detection ignore turn restriction at the boundary of a neighbourhood

If there are turn restrictions at the boundary of a neighbourhood, then the shortcut detection does not

## Example

https://github.com/a-b-street/abstreet/assets/5346065/519aea45-1db8-4bb6-8fac-277fbe061556

The video shows an example of current behaviour whilst editing Elms Drive - Oxford.

* When there are no turn restrictions, two shortcuts are detected for Elms Drive (one going in either direction)
* With turn restrictions from Marsh Lane, it is not possible to have a shortcut starting at the eastern end of Elms Drive, but this is still detected as a possible shortcut.

## Suggested improvement:

The shortcut detection should respect turn restrictions at the boundary of the neighbourhood.