Viewer - allow full screen or custom dimensions (programmatic control of window size)

### Problem Statement
AFAIK it isn't possible to programmatically open a Viewer window so that when it opens up it is _**already**_ in full screen mode. Window dimensions when it opens up seem to be limited to about 60% of screen dimensions (though it is always possible to resize the window manually.)

### Proposed Behaviour
Let me first explain a bit the context. I am developing scripts for proteomics data analysis in R which rely heavily on the interactive environment of RStudio. I rely heavily on Shiny apps to define parameters and interact with the user whenever input is required. In this context, I open each app in Viewer with:
> myApp <- shiny::shinyApp(ui, server, options = list(height = screenRes$height, width = screenRes$width))
> shiny::runApp(myApp)
Because these apps are complex, it does make sense to view them full screen in the Viewer, otherwise often the layout just breaks. It does make for a more seamless user experience if apps open up in a correctly sized window.
Now of course, it is possible to either open the app in browser or to resize the window after opening it, so it really isn't a big deal. But it would be a nice-to-have.

Start by tracing how the Viewer opens a Shiny app from the R entry points shown, shiny::shinyApp and shiny::runApp, into the Viewer window. The issue names no repository files or tests; done would mean that an app can request full-screen or custom dimensions when it opens.

Written by the indexing model from the issue text.