Design LTN tool: Improve isolated cell detection

## Current behaviour:

When placing modal filters on, the tool will warn the user if the filters create a cell which is unreachable by car).

When adjusting one-way streets or turn restrictions, no equivalent warning is given if the edits create an unreachable cell.

## Example

https://github.com/a-b-street/abstreet/assets/5346065/4e032804-f28e-4199-b0d7-c37733a4221a

Video shows current behaviour while editing Demesne Fuze in Oxford

* Add a modal filter - The isolated cell is clearly highlighted.
* Add one-way restrictions that make it impossible to drive north along Demesne Fuze and enter the estate (no warning given).
* Add turn restrictions (no straight ahead) from Demesne Fuze and make it impossible to enter the estate (no warning given).

## Suggested improvement:

The procedure for detecting unreachable cells should be improved to account for oneway street and turn restrictions. This should apply both when importing OSM data and when editing in the LTN tool.