ark: Access the R API from a single thread

[lionel-]

Extracted from #1420.

Currently auxiliary threads like the LSP and the environment threads access the R API from their own thread, different from the one where R is running. This is causing stability issues with reticulate (see discussion in #1420 and #1419) and Shiny (see #431).

To fix this, we should package up accesses to the R API as tasks run on the R thread at interrupt/yield time.

See also different approach outlined in #802

[lionel-]

With posit-dev/ark#109 we now have infrastructure in place to run R code from a single thread.

[jennybc]

🎉