feat: add completions scripts command

[Salnika]

What's the problem this PR addresses?

This PR adds built‑in way to generate shell completions. It adds a yarn completions command that generate a completion script for bash/zsh/fish/PowerShell.

It closes #512
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How did you fix it?

• Added a new completions command to @yarnpkg/plugin-essentials that generates completion scripts for bash, zsh, fish, and PowerShell.
• Added an --install option to write the script into shell completion locations.
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Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.