fix: validate package workspaces field

[wyf027]

Summary

• Treat package.json#workspaces as a workspace marker only when it has a valid npm/yarn shape.
• Accept an array of strings or an object whose packages field is an array of strings.
• Add workspace root detection tests for valid array/object forms and malformed workspaces fields.

Closes #244.

Tests

• cargo fmt --all --check
• cargo test -p vite_workspace

[fengmk2]

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