Warn about outdated ncu + auto update

Having an up to date ncu is essential for folks to land PRs following the latest process, but today if someone tries to use ncu with an older version it will work, which might lead to missing checks before landing something. To avoid that, ncu should check if it's running the latest version and warn the user if it's not (potentially with an option to self-upgrade).

首先定位 ncu CLI 入口点以及任何现有的版本检查或升级测试。定义 ncu 如何检测自身已过时，以及自我升级是否在范围内；完成意味着较旧的 ncu 会在可能错过必要检查之前警告用户。