Change CLI functional test plugin files to ESM

[Harjun751]

Refer to #2836

[gerteck]

Related Snippet:

Maybe we should open an issue to track this chore as well? I can offer to tackle this hopefully small task if needed.

Understand the situation with the tests in cli/tests/functional/
The fix to

add a package.json at the scope of the test sites to let those plugin files be interpreted as CJS instead of ESM.

As you mentioned,

Either way, if there is, the user should already be using ESM syntax then, so they shouldn't encounter a breakage.

We should also be hopefully migrating these test custom plugins in to ESM formats, since we would expect custom plugins to be written in ESM moving forward.

I think I'll put it as a separate PR.

since we would expect custom plugins to be written in ESM moving forward.

I think ideally we should be able to support parsing both, right? If so then maybe we should update file parsing to accept ".cjs" extensions. I'm not too sure though. If we don't need CJS support then I suppose we can just migrate those plugins and call it done.