Switch to ESLint v9 and flat config

[KristjanESPERANTO]

Since PR #3551 was not yet complete, I made my own attempt.

1. Update to ESLint v9.
2. Replace deprecated .eslintrc.json and .eslintignore by flat config eslint.config.mjs.
3. Adapt check_config.js to use flat config.
4. Since eslint-plugin-import still doesn't support ESLint v9 I removed it. We can add it back when it does support v9.
5. Run tests npm run check:js and npm run config:check.
6. In order not to overload this PR, I have not yet activated more additional rules - there are some useful ones in the new plugin @eslint/js.

@bugsounet, please don't take it as an offence that I have created a competing PR. The migration to ESLint v9 has been burning under my nails for some time.

[bugsounet]

Don't worry, I just wanted to start coding the tests with eslint v9.
I know that you are more skilled than me on this ;)

[khassel]

@KristjanESPERANTO

to be honest I have no clue about eslint but with this PR we have

	"dependencies": {
		"eslint": "^9.11.1",
	},
	"devDependencies": {
		"@eslint/js": "^9.11.1",
	},

in package.json, the versions are the same and both npm packages refer to the same github repo.

Is this correct or would be the one under dependencies sufficient?

[KristjanESPERANTO]

eslint is the real ESLint which we need in dependencies because we use it in the config check.

@eslint/js is a plugin for ESLint that we use for linting - so devDependecies is the right place for it.

So everything is fine in my eyes.

[khassel]

Thanks for the explanation, I checked for dependency updates and saw the same update for both ...

[KristjanESPERANTO]

The developers seem to synchronize the version number of both. So we will be seeing this even more often in the future.

I understand the doubt 🙂 They should have found a better name for the plugin.

[khassel]

hm, I tested now without the @eslint/js (see this commit) and all tests were fine (?)

[KristjanESPERANTO]

Yes, it works because the plugin is a dependency of ESLint.

https://github.com/eslint/eslint/blob/main/package.json#L104

Isn't it recommended to put direct dependencies into the package.json? We import the plugin in the ESLint config.