Add support for quarto

[sellorm]

Add explicit support for quarto, so that serving quarto's .qmd documents is equivalent to serving Rmarkdown's .Rmd files.

shiny-server supports a kind of render-on-demand feature for rmarkdown using rmarkdown::run(). We need equivalent functionality for quarto.

Quarto features should maintain parity with rmarkdown features in shiny server where possible.

For anyone else reading this, it's already possible to publish rendered quarto-generated html to shiny-server. This request is solely about the render-on-demand feature.