Add method to get last path cost to AStar2D, AStar3D, and AStarGrid2D

[theashtronaut]

Adds a method to return the last total pathfinding cost to AStar2D, AStar3D, and AStarGrid2D. This method returns the g_score of the last closest point. This enables it to work both with allow_partial_path if the end point was unreachable, or with normal pathfinding. g_score is used instead of f_score as it is more accurate if the path was unreachable but still executed. If the end point was unreachable and allow_partial_path is false, it will return -1. It also returns -1 if a pathfind action has not been performed yet, such as after first instantiating the class.

Closes: godotengine/godot-proposals#5667

Obsoletes: #64150

This PR performed a entire path find operation to return the score, instead of reading it from the last closest point. It was last updated Aug 11, 2022.

Example:

Download: astar_test_pathcost.zip

Video:

astar-path-cost.webm

Blue tiles are 0.5 cost, red tiles are 2 cost, normal tiles are 1 cost. Cost is displayed in a UI element. Towards the end you can see that when clicking on a solid/disabled tile, without partial path the value will return -1 but with it returns the cost of the path closest.