feat(nuxi): support mode flags for `add` command

[Diizzayy]

• [ ] 📖 Documentation (updates to the documentation or readme)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [x] 👌 Enhancement (improving an existing functionality like performance)
• [ ] ✨ New feature (a non-breaking change that adds functionality)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Allow --client|--server flags to be used with nuxi add to specify that the respective entity should be either client / server only.

This PR also handles the case where a user manually specifies a file extension when using nuxi add, With current implementation nuxi add plugin test.ts would result in /plugins/test.ts.ts being generated.

Side note: Nuxt 3 encourages typescript usage for optimal developer experience, hence though different entities allow for varying extensions, nuxi add defaults to (and enforces) .vue and .ts.

📝 Checklist

• [ ] I have linked an issue or discussion.
• [x] I have updated the documentation accordingly.

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