starting R sessions

https://github.com/r-lib/callr/
tests for examples https://github.com/r-lib/callr/blob/feature/session-improvements/tests/testthat/test-r-session.R

```r
library(callr)
# start session
rs <- r_session$new()
# poll - for session to be ready to accept commands
rs$poll_io(-1)
#>   output    error  process
#> "nopipe" "nopipe"  "ready"
```