Checks if the user is on macOS and app is supplied, and uses the 'open'
system command if so. Falls back to fs::file_show() otherwise.
See also
Other build-dev:
as_filename(),
construct_app_metadata(),
copy_files(),
create_rprofile(),
create_rproject(),
get_latest_entry(),
get_valid_opts(),
in_tmp_env(),
open_doc_and_wait(),
set_project_paths(),
str_to_filename(),
truncate_string(),
validate_id(),
validate_status(),
write_app_metadata()