Add an option to toggle visuals in the moving navigation agents benchmarks

[OverloadedOrama]

To make it easier to just check CPU usage of the navigation system itself, without visuals. The visuals are enabled by default, like they are in the physics benchmarks, but I can make them disabled if desired. EDIT: They are now off by default.

[Calinou]

This should be applied to the physics benchmarks as well. I'd also suggest parsing a command-line argument instead of using a variable that needs to be changed manually in the source code, e.g. --debug-visuals.

There's a system parsing CLI arguments into member variables if they exist with a arg_ prefix in main.gd: https://github.com/godotengine/godot-benchmarks/blob/main/main.gd#L5-L10