Improve default visualization_recipe() for estimate_predicted with multiple continuous predictors

[bwiernik]

The current approach doesn't include uncertainty bands. I also find using alpha instead of color to map predictor levels a bit hard to follow.

It would be good to adopt an approach more like sjPlot::plot_model(type = "int") and include a smaller number of lines for specific levels of the second predictor. This would also automatically provide a nice interaction plot.