style inline code and fences

[lorenzwalthert]

As introduced with {roxygen2} release 7.1, the following is evaluated:

#' ```{r}
#' # This block of code will be evaluated
#' summary(iris)
#' ```

# as well as

#' roxygen2 created this manual page on `r Sys.Date()` using R version
#' `r getRversion()`.

Implementation would be similar to what we do for Rmd files.