feat(build-a-box): Add example value box builder app

[gadenbuie]

For #778

To run the app call the following

shiny::runApp(system.file("examples/build-a-box", package = "bslib"))

There's only one external "option" that is controlled by the ENABLE_THEMER envvar. When exactly "true", the themer is included in the app -- it's also minimized on init with a bit of JavaScript (maybe we should add first-class support for controlling how or where the themer UI is inserted).

Sys.setenv("ENABLE_THEMER" = "true")
shiny::runApp(system.file("examples/build-a-box", package = "bslib"))

You can also turn on the themer for a running app by adding ?themer or ?themer=true to the URL and reloading the page.

[cpsievert]

Probably worth adding rstudio/shiny to the Config/Needs/deploy field in DESCRIPTION (so the dropdowns work better in dark mode)

[cpsievert]

So awesome, feel free to merge after the minor suggestions!

[cpsievert]

One other optional nit. I think input_dark_mode() could come out the card and go in the page header (and maybe the same for the code export?)

[gadenbuie]

One other optional nit. I think input_dark_mode() could come out the card and go in the page header (and maybe the same for the code export?)

I like this, thanks for the suggestion!