feat: update repo to use flat config && update integration tests

[codiini]

This PR adds support for ESLint V9 Flat config and also updates the /integration-tests to support both flat-config and legacy-config.

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Latest commit: 0863962a1bb5926965ee73e36558c2b61f6188fb

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[codiini]

@ota-meshi I'm having issues with the Integration tests working on CI.

I got them to work locally using the ESLint class method, but that failed when I pushed to CI. Then I switched to using the execySync node process (mostly copied from the vue eslint plugin) in this commit but that failed locally and also on CI.

Do you know what the issue might be?

[ota-meshi]

Wow! Thank you for this PR!

I haven't run it so I don't know, but we might need the --ext option, as the old ESLint only lints *.js files by default, or specifying the file name directly might solve the problem.

[ota-meshi]

Ah, I think it's because the env isn't being passed and therefore the node path can't be resolved.

-  env: { ESLINT_USE_FLAT_CONFIG: 'false' }
+  env: { ...process.env,  ESLINT_USE_FLAT_CONFIG: 'false' }

[codiini]

@ota-meshi Thank you, I've made the update. The CI still fails for both flat and legacy config but with different errors this time

EDIT: The Legacy config test works now. Debugging the issue with the flat config now