Add onStop function (#1770)

* NEWS item

* added `onStop` arg to `shinyApp()` (and renamed our internal `onEnd` - which is what was calling `on.exit()` already - to `onStop` as well)

* added onStop() function

* add entry for documentation

* make it work for all possible app structures (interactive, saved as app.R, saved as ui.R and server.R)

* fix #1772: make sure `onStart` works in all scenarios

* update NEWS

* improved wording

* more wording

* and more wording

* don't stop execution if a `onStop` callback function results in an error

* remove "(all sessions have been disconnected)" because it's misleading

* add @seealso documentation

* shamefully forgot to Cmd Shift D

* change code place

* Code review feedback

* onStop: use session argument instead of scope

inst/staticdocs/index.r

@@ -202,7 +202,8 @@ sd_section("Utility functions",
     "repeatable",
     "shinyDeprecated",
     "serverInfo",
-    "shiny-options"
+    "shiny-options",
+    "onStop"
   )
 )
 sd_section("Plot interaction",