feat(editor): ":restart" command #33953

Problem:
Developing/troubleshooting plugins has friction because "restarting"
Nvim requires quitting and manually starting again. #32484

Solution:
- Implement a `:restart` command which emits `restart` UI event.
- Handle the `restart` UI event in the builtin TUI client: stop the
  `nvim --embed` server, start a new one, and attach to it.

src/nvim/api/ui_events.in.h

@@ -177,3 +177,5 @@ void msg_history_clear(void)
 
 void error_exit(Integer status)
   FUNC_API_SINCE(12);
+void restart(void)
+  FUNC_API_SINCE(14) FUNC_API_REMOTE_ONLY FUNC_API_CLIENT_IMPL;