fix(tui): wait for embedded server's exit code

Uses the undocumented "error_exit" UI event for a different purpose:
When :detach is used on the server, send an "error_exit" with 0 `status`
to indicate that the server shouldn't wait for client exit.

src/nvim/ui_client.c

@@ -348,6 +348,16 @@ cleanup:
   restart_args = (Array)ARRAY_DICT_INIT;
 }
 
+void ui_client_event_error_exit(Array args)
+{
+  if (args.size < 1
+      || args.items[0].type != kObjectTypeInteger) {
+    ELOG("Error handling ui event 'error_exit'");
+    return;
+  }
+  ui_client_error_exit = (int)args.items[0].data.integer;
+}
+
 #ifdef EXITFREE
 void ui_client_free_all_mem(void)
 {