[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.