Following up on a proposal I made here: godotengine/godot-proposals#11573

I've added a new function get_closest_segment that returns the point ids of the closest segment to a point. This has multiple use cases for a lot of different types of games and gives us more power than the existing function did.

1. It lets us get the normal of the segment
2. It lets us use the tangent to splice the segment
3. It lets us know where to begin path finding within the segment
4. It allows us to snap nodes along the tangent of the path
5. It allows us to interpolate to any position within the segment

For anybody interested in further reading about how to represent roads, particularly curved roads within a city builder, I recommend this blog site:
https://www.redblobgames.com/articles/curved-paths/

• Bugsquad edit, closes: Add AStar2D/AStar3D methods to get the nearest segment point IDs and normal godot-proposals#11573