fix(cli): Auto-install CSS plugin with Sass during init

[pvignau]

This pull request updates the plugin selection logic in the initCmd function to support plugin groups and prevent duplicate installations. The most important changes are:

Plugin selection logic improvements:

• Updated the plugin options in the multiselect prompt so that each option can now represent one or more plugins (e.g., the "Sass" option now installs both @terrazzo/plugin-css and @terrazzo/plugin-sass). This allows for grouped plugin installation.

• Modified the logic for determining which plugins to install by flattening the selected plugin arrays, filtering out already installed plugins, and using a Set to ensure no duplicates are installed.## Changes

Fixes #531

How to Review

1. Initialize a new Terrazzo project:

   npx tz init

   • Select any design system (e.g., "Adobe Spectrum").
   • When prompted to "Install plugins?", select "Sass" but do not select "CSS".

2. The init command will complete successfully, reporting that the plugins were installed.

3. Attempt to build the project:

   npx tz build

It should build !

[changeset-bot]

🦋 Changeset detected

Latest commit: d465304

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 7 packages

Name	Type
@terrazzo/cli	Patch
@terrazzo/plugin-css	Patch
@terrazzo/plugin-js	Patch
@terrazzo/plugin-sass	Patch
@terrazzo/plugin-swift	Patch
@terrazzo/plugin-tailwind	Patch
@terrazzo/plugin-vanilla-extract	Patch

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[drwpow]

Perfect, thank you!