[eslint-plugin-mobx] ESLint v9 Support

[dartess]

ESLint has released v9: https://eslint.org/blog/2024/04/eslint-v9.0.0-released/

Could you please add compatibility with version 9?

As far as I can see, there are quite a lot of breaking changes there. And flat format is now enabled by default. I'm not sure, but this means that you either need to migrate completely to a flat config, or maintain two versions of the config.

[SimenB]

As I was posting this message, the above PR showed up 😀

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ESLint@8 has been deprecated on npm now, would love to see official support here 🙂