Migrate to ESLint 9 and flat configs

[philippfromme]

Proposed Changes

• Migrates this plugin to ESLint 9, ESM, and flat configs.
• Adds tests for when there are no errors and when there are errors (verifying that the configs actually work).
• Improves test organization

Closes #19

Checklist

To ensure you provided everything we need to look at your PR:

• Brief textual description of the changes present
• Visual demo attached
• Steps to try out present, i.e. using the @bpmn-io/sr tool
• Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}

[nikku]

Could you explain how the "errors" / "no errors" is supposed to work? I see that you commented out all errors. Would such a test fail if the line did not yield the particular error?

[barmac]

Smart.

[barmac]

Could you explain how the "errors" / "no errors" is supposed to work? I see that you commented out all errors. Would such a test fail if the line did not yield the particular error?

As far as I understand, the errors specs contain overrides which are reported if eslint does not use them, cf. #20 (review)

So if I have a snippet below, the script will report unused directive:

const life = 42; // eslint-disable-line no-unused-vars
console.log(life);

[nikku]

I spent quite some time trying to dig how eslint@9 handles overrides / globals. My goal was to write a test case that would pass with expect being a global, but fail if we don't declare it.

I propose that we sync (knowledge exchange) on the topic later this week @philippfromme. Seems to be of inherent complexity 🙈

[nikku]

Ok, I went on to drastically simplify this PR. From what it looks like 9be702a is a requirement to properly (later on) be able to have simple, per branch configurations.

@barmac @philippfromme Please review.

[nikku]

What I'd be interested in is whether the "full ES6" style of configuring things has a negative impact in different places (i.e. when consuming in Desktop Modeler).

➡️ I guess not, because we can provide the configuration in an mjs file, as you propose.

[barmac]

LGTM 🚀

[barmac]

One more thing [required]

[barmac]

OK done via decea27. Check out the comment if you want to know details.

I'd say we can squash it now.

[nikku]

All cleaned up and ready to merge 🚀

[barmac]

Congratz @philippfromme and @nikku for a successful migration

[philippfromme]

Thanks for the help, guys! 💪🏻