Call unlist(strsplit(x, '\n')) on the character vector x and
make sure it works in a few edge cases: split_lines('') returns
'' instead of character(0) (which is the returned value of
strsplit('', '\n')); split_lines('a\n') returns c('a', '') instead of c('a') (which is the returned value of
strsplit('a\n', '\n').