Add debug visualizer

[Razoric480]

A special Node2D/Spatial/PackedScene that can be added to a scene and fed an agent and/or behavior, with the ability to draw whatever information is relevant or selected by the programmer.

• Arrows for directions
• Lines for raycasts
• PolyLines for paths
• Circles for predicted target positions
• Numerical data, like current velocities and accelerations

[NathanLovato]

This should be part of https://github.com/GDQuest/godot-gdquest-theme/ or another repo for drawing utilities and debug tools I think, we could use those things in other projects, and we have some debug panel already to monitor values ingame on the screen.