Change Confidence Interval in interact_plot

I am using interact_plot to create interaction plots and want to change the size of the confidence interval. However, int.width is not changing the width of the confidence intervals. The images below show what is happening.

Looking below, we can see how the graph looks with int.width = 0.95

In the next image, we can see when I set the int.width = 0.4

In addition, here is the code used to create the graphs in the images.
```r
# create the model object
model <- lm(mpg ~ cyl * hp, data = mtcars)

# create the interaction plot used in the first image, 95% confidence interval
interact_plot(model, pred = cyl, modx = hp, interval = TRUE, int.width = 0.95)

# create the interaction plot used in the second image, 40% confidence interval
interact_plot(model, pred = cyl, modx = hp, interval = TRUE, int.width = 0.4)
```

Start by running the supplied R examples with interact_plot and compare how int.width is handled for the two confidence intervals. Trace the interact_plot entry point and its interval calculation, then add a regression test showing that 0.95 and 0.4 produce different interval widths.

Written by the indexing model from the issue text.