feat: Shiny anti-pattern

[schloerke]

Ex:

• Do not define reactive() within other reactive() or observe() calls
• Prefer reactive() calculations over reactiveValues()+observe()
• Do not use global variable state within reactive expressions

Related:

• https://github.com/rstudio/ShinyDeveloperConference/blob/45737f1f3386fb3a9194c40f9a999b58f60971de/Reactivity/slides.Rmd#L303-L314
• https://github.com/rstudio/ShinyDeveloperConference/blob/45737f1f3386fb3a9194c40f9a999b58f60971de/Reactivity/slides.Rmd#L440-L488