fix: Use mandatory file extensions

[modernizer-bot]

A file extension must be provided when using the import keyword to resolve relative or absolute specifiers. Directory indexes (e.g. './startup/index.js') must also be fully specified.

This behavior matches how import behaves in browser environments, assuming a typically configured server.

Ref: https://nodejs.org/docs/latest-v17.x/api/esm.html#mandatory-file-extensions

(Even if you perhaps using require() and extension-less paths, using extension can help with the migration to esm later)

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