Support ESLint's flat config

[AriPerkkio]

Check whether anything special is required for ESLint's flat config support. It might already work out of the box. At minimal, add test cases.

• https://eslint.org/docs/latest/user-guide/configuring/configuration-files-new
• https://eslint.org/blog/2022/08/new-config-system-part-3/#using-flat-config-with-the-eslint-class