Chore: use explicit path (extension)

[jimmywarting]

This could theoretically work without the need of compiling things, just need to add the extension

Mandatory file extensions#
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/dist/latest-v17.x/docs/api/esm.html#mandatory-file-extensions

[MichaelDeBoey]

Hi @jimmywarting!

Since this repo is unmaintained, you might want to re-open this PR in the @eslint-community fork https://github.com/eslint-community/eslint-utils

For more info about why we created this organization, you can read https://eslint.org/blog/2023/03/announcing-eslint-community-org