refactor(message): propagate highlight id instead of attrs

Problem:  Highlight group id is not propagated to the end of the message call
          stack, where ext_messages are emitted.
Solution: Refactor message functions to pass along highlight group id
          instead of attr id.

src/nvim/input.c

@@ -55,7 +55,7 @@ int ask_yesno(const char *const str, const bool direct)
   int r = ' ';
   while (r != 'y' && r != 'n') {
     // same highlighting as for wait_return()
-    smsg(HL_ATTR(HLF_R), "%s (y/n)?", str);
+    smsg(HLF_R + 1, "%s (y/n)?", str);
     if (direct) {
       r = get_keystroke(NULL);
     } else {