Add simultaneous intervals on smooths in `draw.gam()`

[gavinsimpson]

Discussed in #313

^{Originally posted by AlainZuur September 6, 2024}
Hello...what would be the easiest way to get simultaneous CIs with draw()? I can get them manually with confint...but it would be much easier if draw() could plot them (I don't think it can??).

I can see how to do it manually:
https://gavinsimpson.github.io/gratia/articles/custom-plotting.html
This part:
sm <- smooth_estimates(m) |> add_confint()
And then replace by: confint and adjust the type argument. Is there any way of doing this faster?

Or change that line in draw.gam?
Alain