Add warning for private fields with [Export] - unexpected serialization/display behavior

[van800]

public abstract class Parent : Node
{
    [Export] NodePath testPath = new NodePath();

    public override void _Ready()
    {
        GD.Print("Parent Ready");
        GD.Print($"Is Path empty? {testPath.IsEmpty()}");
    }
}

public class Inherited : Parent
{
    [Export] NodePath testPath = new NodePath();

    public override void _Ready()
    {
        base._Ready();
        GD.Print("Inherited Ready:" + testPath);
    }
}

godotengine/godot#42244