Fix ESM support in Node 22+ when the package includes the main property

[marcalexiei]

Try to close #615

This PR adds a check in the loadPlugin require inside package-json.ts to verify if the returned value is an ESModule, in an attempt to resolve #615.

Warning

I was unable to create a fixture inside test/fixtures to add a test to avoid regression.
I created a new fixture named esm with a package.json of type module, including main and exports entries.
However, when running jest, the require call consistently threw an error, making my code unreachable.

---

In test/lib/generate/cjs-test.ts,
within the 'package.json main field points to a non-existent file' test,
I updated the FIXTURE_PATH generation to align with the pattern used in other tests.

[bmish]

Thank you for the PR.

Have you done some manual testing to verify if this improves the behavior and avoids regressions?

I understand some of these scenarios can be difficult to test with our testing setup. You may need to add a istanbul ignore comment for untestable lines to pass CI.

[marcalexiei]

Have you done some manual testing to verify if this improves the behaviour and avoids regressions?

Yes, I tested the patch in the repository provided in the issue reproduction.
I applied the patch directly on node_modules/eslint-doc-generator/dist/lib/package-json.js.
I tested using the following node versions:

• v20.18.2
• v22.14.0

---

I understand some of these scenarios can be difficult to test with our testing setup. You may need to add an istanbul ignore comment for untestable lines to pass CI.

Done, now it should pass.

[bmish]

Thanks. What would be a good user-facing title for this fix? E.g. "Fix ESM support in Node 22+"

[marcalexiei]

This bug occurs only when the main property is set, so I propose:

Fix ESM support in Node 22+ when the package includes the `main` property

[bmish]

Thanks!