Have page_navbar() default to fillable = TRUE

[cpsievert]

Partially addresses #569

Note that, although shiny::navbarPage() passes it's arguments to page_navbar(), this change shouldn't impact most existing navbarPage() code since the fillable = TRUE logic is implemented in CSS that only comes from bslib (thus, you need theme = bs_theme() to activate it):

# The `theme = bs_theme()` is needed in order for fillable = TRUE to work
navbarPage(title = "foo", fillable = TRUE, tabPanel("Plot", plotly::plot_ly()), theme = bs_theme()) |>
  browsable()