Better RStudio detection

[gaborcsardi]

Using the new env vars.

Closes #465.

[gaborcsardi]

FYI @hadley @romainfrancois

To test, you can run print(cli:::rstudio_detect()) in a test file, in a job, in a knitr chunk like below, etc.

```{r R.options = { print(cli:::rstudio_detect()); list()} }
"foobar"
```