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

首先定位 plugins 命令的实现，以及文档中对 ns plugin install 的所有引用；该 issue 未提供文件路径或测试路径。确认项目预期的弃用行为，更新引用以使用包管理器的措辞，并验证命令和文档是否一致地反映了这一决定。