fix: rootDir calculation should ignore declaration files

[Timeless0911]

Summary

rootDir calculation should ignore declaration files

see https://www.typescriptlang.org/tsconfig/#rootDir

Related Links

closes: #385

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[codspeed-hq]

CodSpeed Performance Report

Merging #389 will not alter performance

_{Comparing fix/rootdir (e329398) with main (ce4dea9)}

Summary

✅ 5 untouched benchmarks