This PR closes numerous issues with tabsetPanel()/navlistPanel()/navbarPage(). See the respective issue links for examples:

1. !!! may now be used to splice tabPanel()s into them (closes tabsetPanel() doesn't work with list of panels #2927)
2. The tabsetPanel() and navlistPanel() functions gain header/footer arguments, akin to the already existing header/footer arguments in navbarPage(), making it easier to include content that should appear on every tab (closes Make it easier to include header/footer content in tabsetPanel()/navlistPanel()  #3322).
3. NULL values are now dropped instead of producing an empty nav item (closes tabsetPanel doesn't ignore NULLs #1928).
4. More informative error when non-tabPanel()/shiny.tag objects are supplied to ... (closes tabsetPanel() et al give confusing error messages when they receive incorrect input #3313, closes tabSetpanel first argument #1823).
5. New informative warning when shiny.tag object(s) are supplied to .... In this case we will continue to create an "empty" nav item and include the content on every tab, but the warning will mention the (new) header/footer args, which is likely what the user wants (closes Tab panel behavior when shiny tags are supplied  #3321).
6. Bootstrap 4+ HTML/CSS markup is now produced when used with theme = bslib::bs_theme() (closes Tab panel functions don't produce Bootstrap 4 compatible markup #3320). At least for now, you'll still want {bslibs}'s BS3 compatibility layer to provide sensible styles in navlistPanel() and navbarPage(), but perhaps in follow up PRs we can explore completely removing the compatibility layer dependency for those as well. The main downside to this change in behavior is that tabsetPanel() now returns tagFunction()s, meaning that user code which makes assumptions about the return value may break, but hopefully by providing more intelligent tag modification functions (see, for example, Add tagFunction() awareness to tag modifying functions htmltools#202), this won't be much of an issue.

Testing notes

Install remotes::install_github("rstudio/shiny") then verify there is no regression in the example app above as well as the one below (in the app below, when you click add 'dynamic' tab, a tab should be inserted between Static1 and Static2 in the dropdown)

library(shiny)
library(bslib)

ui <- fluidPage(
  theme = bs_theme(),
  sidebarLayout(
    sidebarPanel(
      actionButton("add", "Add 'Dynamic' tab"),
      actionButton("remove", "Remove 'Foo' tab")
    ),
    mainPanel(
      tabsetPanel(
        id = "tabs",
        tabPanel("Hello", "This is the hello tab"),
        tabPanel("Foo", "This is the foo tab"),
        navbarMenu(
          "Static", 
          tabPanel("Static 1", "Static 1", value = "s1"),
          tabPanel("Static 2", "Static 2", value = "s2")
        )
      )
    )
  )
)
server <- function(input, output, session) {
  observeEvent(input$add, {
    insertTab(
      inputId = "tabs",
      tabPanel("Dynamic", "Dynamic"),
      target = "s2"
    )
  })
  observeEvent(input$remove, {
    removeTab(inputId = "tabs", target = "Foo")
  })
}

shinyApp(ui, server)