feat(ui): "append" parameter for "msg_show" UI events

Problem:  Consecutive "msg_show" events stemming from an `:echon`
          command are supposed to be appended without a newline, this
          information is not encoded in the "msg_show" event.
Solution: Add an "append" parameter to the "msg_show" event that is set
          to true to indicate the message should not start on a new line.
Considered alternative: Emit a newline for the common case instead at the
start of a new message. That way UIs can more closely follow the logic
as it is implemented for the message grid currently. This would be a
breaking change. The "append" parameter seems OK.

src/nvim/eval.c

@@ -7863,6 +7863,7 @@ void ex_echo(exarg_T *eap)
       char *tofree = encode_tv2echo(&rettv, NULL);
       if (*tofree != NUL) {
         msg_ext_set_kind("echo");
+        msg_ext_append = eap->cmdidx == CMD_echon;
         msg_multiline(cstr_as_string(tofree), echo_hl_id, true, false, &need_clear);
       }
       xfree(tofree);