Runtime Version Scanner: Turn the app into a function within the package that can be exported, minimizing the top level entrypoint app.R

[toph-allen]

A suggestion from @karawoo in #279 to make the top-level app.R a simple function call.

I feel that we should proceed with this or not depending on whether we think it's easier for people new to the codebase to comprehend the organization of the app. It might be better to stick more closely to "shiny app" conventions than "R package" conventions.

[m20m22]

It might be better to stick more closely to "shiny app" conventions + 1