Deprecate `plugins` command

Is your feature request related to a problem? Please describe.

No response

Describe the solution you'd like

To simplify, per discussion with @rigor789 in Discord.

Describe alternatives you've considered

Devs can use their package manager directly to install plugins: npm install <foo>, etc.

Anything else?

I'd suggest any reference to ns plugin install in docs be changed to only mention NPM's install command: "npm install <foo> or as appropriate for your package manager", to reduce docs maintenance. Users of other package managers know the analogous command for their package manager.

Please accept these terms

• I have searched the existing issues as well as StackOverflow and this has not been posted before
• I agree to follow this project's Code of Conduct

先找出 plugins 命令的實作，以及文件中所有對 ns plugin install 的參照；此 issue 未提供檔案或測試路徑。確認專案預期的棄用行為，更新參照以使用套件管理器的措辭，並驗證命令與文件是否一致地反映這項決定。