Deprecate `plugins` command

[jasongitmail]

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

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