Make `is_mac()` helper function

[eroten]

We have this bit of code scattered across the package, and could make it more succinct with a function like

is_mac <- function(){
  grepl("mac", osVersion)
}

returns a logical, TRUE or FALSE.

Or this may be overkill for a 23 character function. 🤷

[ericrobskyhuntley]

Appears to have been implemented in 9d6e4ed ? Close issue!