refactor(index): use path.dirname over regex to resolve bin path

[Fdawgs]

Purpose-built path utility should be much more robust than my regex!

Checklist

• Run npm test
• Commit message adheres to the Conventional commits style, following the @commitlint/config-conventional config

[Copilot]

Pull request overview

Refactors how the UnRTF binary installation directory is derived from which/where output by using Node’s path.dirname() instead of a custom regex, aiming for more robust path handling.

Changes:

• Import dirname from node:path.
• Remove the cached UNRTF_PATH_REG regex used to parse which/where output.
• Compute #unrtfPath via dirname() of trimmed which/where stdout, and adjust the win32 fallback condition accordingly.

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[Copilot]

if (which) { this.#unrtfPath = dirname(which); } will also treat any non-empty diagnostic message on stdout as a path (e.g., some where/which failure outputs), resulting in dirname(...) === "." and a misleading later "Unable to determine UnRTF version" error. Consider gating this assignment on spawnSync(...).status === 0 and/or validating that the selected line is an absolute/expected executable path before setting #unrtfPath.

[Copilot]

The constructor logic now depends on parsing where/which output with trimming + dirname(), but the existing tests only cover empty stdout and a single-path stdout. Add a unit test that mocks win32 where returning multiple lines and/or a non-empty "not found" message, to ensure #unrtfPath is set (or remains empty to trigger the win32 fallback) correctly.

[Copilot]

where on Windows can return multiple matching paths (one per line). Calling dirname() on the raw multi-line string will produce an invalid path and can prevent the win32 fallback from running. Consider splitting stdout by newlines and selecting the first valid path (or the first line that ends with unrtf/unrtf.exe) before applying dirname().