Require larger device size to expand navbar toggle; vertically center align toggle icon

[cpsievert]

With this PR,

• The collapse toggle for collapsible navbars (e.g., page_navbar(collapsible = T) and navset_bar(collapsible = T)) will now apply at the lg instead of sm breakpoint.
• The collapse toggle (as well as the title is now guaranteed to be vertically centered)

For example, shiny::runApp("inst/examples/flights"):

Before

After

[gadenbuie]

I like this from the perspective of introducing a minimal change, but this feels like a step in the wrong direction -- going further down a path of introducing BS3 dependencies into BS5 -- rather than moving in the direction of decoupling our navbar markup from legacy uses.

This approach requires a Sass variable to customize the breakpoint, but if we weren't tied to BS3's markup, collapsible could take a string indicating the desired breakpoint and we could use the classes provided in BS5. To me, the collapse breakpoint seems like an app-dependent consideration.

That said, collapsing the navbar at md is consistent with other uses of BS5 in our ecosystem (both Quarto and pkgdown collapse at md). And if we have to pick one, this is the one I'd pick.

[cpsievert]

For posterity, I agree 100% with what you’ve said, but the unfortunate reality is that moving things in the “right” direction (e.g., getting rid of the awkward @extend and/or moving away from .navbar-header, which doesn't exist anymore) would be a fair amount of work (not only in bslib, but rmarkdown, flexdashboard, etc), and we’re not really making the current bs3compat situation any worse off with this change