Update Shiny app template based on bslib

[olivroy]

Relating to #10542

Would it be a good idea to update the app.R template to use more bslib and more modern theming by default and maybe include references inside like to https://mastering-shiny.org/.

Maybe the example in there would be a good start?

https://rstudio.github.io/bslib/articles/dashboards/index.html#hello-dashboards

bslib seems to have improved the look of the resulting app. I think it would be great to integrate that for people who are getting started with shiny for R?

Thanks

Related to https://www.garrickadenbuie.com/blog/shiny-new-bslib-snippet/