Set RTF encoding to UTF-8

The need for this was exposed by the use of a proper ellipsis for `...` in tibble's format method.

As in:

#> # … with 140 more rows

R/reprex.R

@@ -398,7 +398,7 @@ reprex_highlight <- function(rout_file, reprex_file, arg_string = NULL) {
   arg_string <- arg_string %||% highlight_args()
   cmd <- paste0(
     "highlight ", rout_file,
-    " --out-format=rtf --no-trailing-nl",
+    " --out-format=rtf --no-trailing-nl --encoding=UTF-8",
     arg_string,
     " > ", reprex_file
   )