Utility function for watching scss files and pushing updates

If someone has a Shiny application and wants to test out changes to a scss file with live changes, it would be useful to have a function that does something like this (I probably don't have all the details right, but this is the general idea):

r <- reactiveFileReader(500, session, "mytheme.scss", function(file) {
  file
})

observe({
  mytheme <- sass::sass_file(r())
  # If we had a reactive getCurrentTheme(), that would be good here
  theme <- bs_bundle(getCurrentTheme(), mytheme)
  session$setCurrentTheme(theme)
})

In the case where someone uses a Bootswatch theme, this would be really helpful, since the real-time themer in bootstraplib doesn't work well with the way variables are used in Bootswatch.

Start by examining the existing real-time themer in bootstraplib and the reactiveFileReader, sass::sass_file, bs_bundle, getCurrentTheme(), and session$setCurrentTheme() entry points mentioned in the issue. Determine how a watched SCSS file could trigger recompilation and theme updates for Bootswatch variables, then add coverage for the intended live-update behavior.