fix(index): hide spawned unrtf console window on windows

[Fdawgs]

On Windows, spawn may briefly flash a console window for the child process.
Teeny tiny performance boost on Windows OS as the window isn't visible/created.

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 updates the UnRTF wrapper to prevent a spawned unrtf child process from flashing a console window on Windows by using Node’s windowsHide spawn option.

Changes:

• Add windowsHide: true to the spawnSync() call that locates unrtf via where/which.
• Add windowsHide: true to the spawnSync() call that reads unrtf --version.
• Add windowsHide: true to the spawn() call used by convert().

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