feat: Better error message for flat config plugins

[nzakas]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[x] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[x] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

Made it so that if the plugins key is defined as an array, then we throw a specific validation message that lets people know it's an eslintrc style config that needs to be converted. I also added a section on the config migration guide to include details on using FlatCompat.

Refs #17370

Is there anything you'd like reviewers to focus on?

Does the wording make sense?

[netlify]

✅ Deploy Preview for docs-eslint ready!

Name	Link
🔨 Latest commit	999ae7b
🔍 Latest deploy log	https://app.netlify.com/sites/docs-eslint/deploys/64c01b13460be10008ec83fd
😎 Deploy Preview	https://deploy-preview-17399--docs-eslint.netlify.app/use/configure/migration-guide
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[mdjermanovic]

Flat config schema throws this error on any array (it doesn't check whether it's an array of strings), so if the user specifies plugins like this:

module.exports = [{
    plugins: [somePluginObject]
}];

The printed message will look like this:

Oops! Something went wrong! :(

ESLint: 8.45.0


A config object has a "plugins" key defined as an array of strings.

Flat config requires "plugins" to be an object in this form:

    {
        plugins: {
            [object Object]: pluginObject
        }
    }

Please see the following page for information on how to convert your config object into the correct format:
https://eslint.org/docs/latest/use/configure/migration-guide#importing-plugins-and-custom-parsers

If you're using a shareable config that you cannot rewrite in flat config format, then use the compatibility utility:
https://eslint.org/docs/latest/use/configure/migration-guide#using-eslintrc-configs-in-flat-config

Perhaps we should have two templates?

• plugins is an array of strings (or empty array?) - possible eslintrc config, use this template
• plugins is an array but not array of strings - use another template.

[nzakas]

I think for now I'd like to keep things simple and just assume that when people are using an array then they are most likely using an eslintrc-style config. We can always expand later if we find that's not the case.

I've updated to check if the first item in the array is a string to make sure that the message makes sense.

[aladdin-add]

LGTM.

[aladdin-add]

There is a conflict that needs to be resolved.

[mdjermanovic]

LGTM when merge conflicts are resolved.

[fasttime]

Just a typo, and a merge conflict that needs to be fixed. Otherwise LGTM.

[nzakas]

Should be all set. Please double-check that the merge makes sense.