`navs_pill_card(placement = "below")` should use dropup `nav_menu()`

When nav_menu() is used in navs_pill_card() with placement = "below", the menu can sometimes be rendered as a dropdown menu that overflows the card container but isn't visible without scrolling the container.

We should catch this and use a dropup menu instead, which would also result in better looking menus.

Note: this seems to happen only if there isn't anything affecting the height of the card. In general, Bootstrap will detect the overflow and treat the dropdown as a dropup, but there are definitely edge cases where that detection will fail.

library(htmltools)

link_shiny <- tags$a(shiny::icon("github"), "Shiny", href = "https://github.com/rstudio/shiny", target = "_blank")
link_posit <- tags$a(shiny::icon("r-project"), "Posit", href = "https://posit.co", target = "_blank")

navs_pill_card(
  nav(title = "One", p(lorem::ipsum(2))),
  nav(title = "Two", p(lorem::ipsum(2))),
  nav_spacer(),
  nav_item(link_shiny),
  nav_menu(
    title = "Other links",
    align = "right",
    nav("Three", p("Third tab content")),
    nav_item(link_posit)
  ),
  placement = "below"
)

Reproduce the provided R example, then trace the navs_pill_card() and nav_menu() entry points to see how placement="below" is rendered. Done means the menu consistently uses Bootstrap's dropup behavior in this case and no longer overflows the card container.