Add support for flat config (eslint.config.js)

[rakleed]

From the next version 9, ESLint will deprecate the current eslintrc format, and by default will use the new flat config format (eslint.config.js). Already from version 8.21.0 you can use the flat config format. But your README doesn't say anything about how to configure in flat config, so I'm guessing you don't support it.

Guide to migrating a plugin to a flat config: https://eslint.org/docs/latest/extend/plugin-migration-flat-config

[hanc2006]

It would be nice to have a reference example.