terminal: flush vterm output buffer on pty output #8486

Fixes #4151

libvterm uses an "output buffer" for terminal reporting
(e.g. \e[6n to report cursor position)
Flush it in on_channel_output() not just terminal_send_key()

See also this line from pangoterm:
https://bazaar.launchpad.net/~leonerd/pangoterm/trunk/view/607/pangoterm.c#L2159

src/nvim/channel.c

@@ -602,6 +602,7 @@ static void on_channel_output(Stream *stream, Channel *chan, RBuffer *buf,
   // process_channel_event will modify the read buffer(convert NULs into NLs)
   if (chan->term) {
     terminal_receive(chan->term, ptr, count);
+    terminal_flush_output(chan->term);
   }
 
   rbuffer_consumed(buf, count);