Issue with RoadContainer unsnapping 4.x

[TheDuckCow]

See the demo video here, where after snapping in a RoadContainer, the unsnapping red line doesn't appear and we get lots of error messages. While moving it around, the connection that should have shown the red line continues to be "unavailable", but the other open connections that hadn't been used before do show connection hints. My guess is something in the export array vars isn't being set/unset at the right time. This is not a problem in godot 3.x, so we can try basing a fix directly off the godot4 branch.

Let's make sure this works as intended in 4.2 and 4.3, Patrick can do testing in other versions and decide what to do if not working as hoped.

Godot 4.3 example:

unsapping.issues.4.3.mp4

Sample "T-junct" scene:
t_junct.tscn.zip

And as a counter point, see how it's working in Godot 3.5:

demo.godot3.5.mp4

[TheDuckCow]

Putting this as a higher priority as the snapping feature is largely not useable due to not being able to disconnect them like this.