feat: migrate to ESLint v9 with flat configuration

[outslept]

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Transition from ESLint v8 to v9 using the new flat configuration format. Convert .eslintrc.json to eslint.config.js and incorporate .eslintignore paths directly into the configuration.

Note: After migrating to ESLint 9, the linter reports 56 errors that weren't present with ESLint 8. This is likely due to changes in recommended rule sets or rule implementation in the new version.

The migration itself is working correctly (e.g. tools like this https://eslint.org/blog/2024/04/eslint-config-inspector/ are working fine), we need to decide how to handle these new linting errors before merging.

Coming from #406