Navigation Agent distance_to_target_along_nav_path

[RudyFisher7]

I suppose this could work for nav agent 2d as well.

There are a lot of applications where you want to calculate the distance along the navigation path of a nav agent. I think if the node had this feature built-in so that it works with the navigation server, that would be a great addition.

Some applications for this feature:

• Gridless tactical games (like the movement during combat in Divinity: Original Sin 2) where units have a maximum distance they can travel each turn, and the path is required to be visualized ahead of time.
• Simulations where you want agents to stay within a certain range while AI drives their movement behaviors.
• I'm sure there are more, but those two come to mind.

Loving what I'm seeing in this engine.