Add a way to filter neighbor points to AStar2D/3D

[Chaosus]

This PR provides a way for user to control which point needs to be processed or skipped by AStar2D/3D classes.
This can be used for various effects, including simulation of DiagonalMode from AStarGrid2D.

For example, if that AStar2D represents a 2D grid of the 256x256 dimension, the solution to orthogonal movement would look like this:

extends AStar2D
class_name AStarTest

var size = 256 # use your array dimension

func _filter_neighbor(from_id: int, neighbor_id: int) -> bool:
	var fx = from_id % size
	var fy = from_id / size
	
	var nx = neighbor_id % size
	var ny = neighbor_id / size
	
	if fx == nx || fy == ny: # only orthogonal movement is enabled (even if diagonals are connected).
		return false # allow this point
	
	return true # skip this point