Make rhino features availables in Positron IDE

[davidrsch]

Hello, love working with rhino and there are some features I miss from RStudio now that I am transitioning to using Positron IDE:

• Addins, please consider adding snippets to shiny vs code this could be an alternative to addings like module creation.
• Consider modifying comment in app.R file from:

  Rhino / shinyApp entrypoint. Do not edit.

  to:

  Rhino / shiny app entrypoint. Do not edit.

  so positron IDE detect that this files correspond to a shiny app, allowing run shiny app as default for execution button.