Prioritize proximity to green arrow when selecting overlapping transitions in AnimationNodeStateMachine

[vaner-org]

Addresses usability problem mentioned in #10976.

https://github.com/user-attachments/assets/b93f7f5b-e41f-4ccd-afcd-e19f8a856197

When transition lines are overlapping, selection between them is undefined, leading to some guesswork on which transition will be selected. Using proximity to the green arrow always present at the midpoint of each line as bias makes selection much more predictable.

https://github.com/user-attachments/assets/e02c8015-5b7b-45f1-a00b-fc9d0c282e9f

This PR is a companion to https://github.com/godotengine/godot/pull/98402, which improves visibility when selecting nodes and transitions in complex AnimationNodeStateMachine cases.

As this is my first PR, criticism and feedback would be very much appreciated.