fix(modem): Refine mode switch data->command

* netif.stop() moved after setting the transition callback
* send PPP escape sequence if enabled before waiting for transition to
complete
* add newline character before sync() command (after the "+++")

Closes https://github.com/espressif/esp-protocols/issues/692

components/esp_modem/include/cxx_include/esp_modem_dce_module.hpp

@@ -77,7 +77,9 @@ public:
                 if (set_command_mode() == command_result::OK) {
                     return true;
                 }
-                Task::Delay(1000); // Mandatory 1s pause after escape
+                // send a newline to delimit the escape from the upcoming sync command
+                uint8_t delim = '\n';
+                dte->write(&delim, 1);
                 if (sync() == command_result::OK) {
                     return true;
                 }