Add session start/stop events

[trestletech]

Adds session_start and session_stop events.

I originally hoped to install the session_start handler in initialize.R next to the stop handler. Unfortunately, we don't initialize_session_state (which is necessary to get the tutorial ID/name/etc) until we run a session$registerDataObj handler. I couldn't find a viable way to defer the session start logic until the handler had been run, so I went this route instead. (later::later didn't do it late enough).

Listening on http://127.0.0.1:6924
com.rstudio.primers.02-vis-basics (1): jeff
event: session_start
com.rstudio.primers.02-vis-basics (1): jeff
event: section_skipped
com.rstudio.primers.02-vis-basics (1): jeff
event: question_submission
com.rstudio.primers.02-vis-basics (1): jeff
event: session_stop

PR task list:

• Update NEWS
• NO - Add tests (if possible) -- deemed unfeasible?
• N/A - Update documentation with devtools::document()