feat: support bun global packages

[halkn]

Summary

• Add type = "bun" for globally installed npm registry CLI packages.
• Introduce a package manager definition layer so npm-style managers can define their install/update commands in one place.
• Resolve npm registry package versions through registry metadata for both npm and Bun package manager types.
• Update README examples and tests for npm/Bun package manager behavior.

Validation

• uv run pytest (154 passed)
• uv run ruff check src tests
• uv run ty check src tests

Notes

This keeps current npm package usage scoped to public CLI tools, so direct npm registry metadata lookup is acceptable for the existing use case.