refactor(index): rename constructor param to reflect usage

[Fdawgs]

Checklist

• Run npm test
• Documentation has been updated and adheres to the style described in CONTRIBUTING.md
• Commit message adheres to the Conventional commits style, following the @commitlint/config-conventional config

[Copilot]

Pull request overview

This PR refactors src/index.js to rename the UnRTF constructor parameter and internal private field to reflect that the value represents the directory containing the unrtf binary (not the full binary filepath).

Changes:

• Rename constructor parameter from binPath to binDir.
• Rename internal private field from #unrtfPath to #unrtfDir.
• Update JSDoc to describe the parameter and path getter as a directory path.

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