Improve navset_pill_list() width and styles

[gadenbuie]

As described in posit-dev/py-shiny#1112, the layout and width of navset_pill_list(), at least when using the function defaults, could be improved.

[corey-dawson]

In addition, should include

1. max-width for the vertical navigation menu (the part with the nav items). Should scale similar to sidebar
2. total width of the widget (already exists)
3. height: I don't want the widget changing sizes per nav item selected. Widget should have a set height property so each output pane is the same height (currently have to create a div with a set height for each nav item panel to achieve a clean height)
4. I would like to use this as a sidebar sometimes. instead of having a sidebar with inputs, have this as a sidebar with navigation objects. Anything to make this more streamlined. Also, having the ability to add other objects below the nav items like a button could be useful

[gadenbuie]

4. I would like to use this as a sidebar sometimes. instead of having a sidebar with inputs, have this as a sidebar with navigation objects. Anything to make this more streamlined. Also, having the ability to add other objects below the nav items like a button could be useful

Thanks! We've been planning to implement sidebar navigation for a while and are tracking that work and relevant suggestions in #585.