add eslint@9 support and flat config

[dartess]

Code change checklist

• Added/updated unit tests
• Updated /docs. For new functionality, at least API.md should be updated
• Verified that there is no significant performance drop (yarn mobx test:performance)

Closed #3856

• added compatibility with eslint@9;
• added support for flat config;
• added recommended config in flat format;
• added documentation for using with flat config;
• added tests to check rules in legacy (using eslint@7) and in modern (eslint@9) environment.
• everything is tested on my own project with flat config;

[changeset-bot]

🦋 Changeset detected

Latest commit: 0d03c33

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
eslint-plugin-mobx	Patch

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[iChenLei]

I'm sorry, I accidentally clicked the button and removed the Draft mode

[dartess]

@iChenLei moved to draft to add changeset; already done

upd: no, still a draft, I'll try to fix the tests in CI.

[dartess]

I think that's all 🙌

[urugator]

There is a preview directory, that provides "live" (you need to reload window in between changes) IDE preview . This is super useful when developing a rule (if you know of better way, let me know). Since you removed "eslint": "^7.0.0" from dev deps, I assume the IDE will use eslint installed at the root of the mobx repo, which is "eslint": "^6.8.0", . Can we make sure it uses 7 or 9? I think we don't need preview for both at the moment.

[dartess]

There is a preview directory, that provides "live" (you need to reload window in between changes) IDE preview . This is super useful when developing a rule

Wow! I didn't realize this was used during rules developing. This looks nice!

if you know of better way, let me know

In fact, I see a flaw in this way. It is essentially a manual way of testing. It can be really convenient during development, but at the end of development, all preview examples (in a good way) should be copied to test cases. However, this can be forgotten, and all this is code duplication (what is bad).

Usually I've seen a simpler way - development through test cases in observation mode. I can add something like "test:watch" to the scripts. It's not as convenient, but it just works and doesn't require a "preview" dir at all.

I assume the IDE will use eslint installed at the root of the mobx repo, which is "eslint": "^6.8.0"

Yes, that's true, at least for JetBrains IDE. I didn't take that into account either.

Option 1: remove "preview" folder for development and switch to tests and watch mode (to avoid the need to synchronize preview and __tests__) (I prefer this)

Option 2: configure preview with eslint@7 (just return it to how it was)

Option 3: configure preview with eslint@9 (all versions below 9 are officially deprecated now, so this makes sense, but there may be problems with switching between different configs for editors and IDE)

Your opinion?

[urugator]

It is essentially a manual way of testing

Let me aswer with some rhetorical questions:
Have you ever developed an application just by writing tests?
At the time of writing, eslint supported multiple suggestions/fixes, but vscode couldn't handle that, how would you know?
In the OP, you're mentioning "everything is tested on my own project with flat config;". So automated tests did not suffice?

Your opinion?

I am fine with both 2/3 as long as it works in vscode :D Your call.

[dartess]

yes, these are good questions.

I rollback eslint@7 as dev deps, now it should work as before.

[urugator]

Thank you for looking into it. Much appreciated.