[eslint-config-moon] Add FlatConfig export

[BinToss]

ESLint's FlatConfig format cannot import legacy eslintrc configs.

I recommend the existing exports remain as-is to allow backwards compatibility until ESLint drops legacy "eslintrc" configs. At which point, eslint-config-moon should also drop eslintrc compatibility. Additional exports should be added to allow users to import the FlatConfig equivalents of the existing exports.

https://eslint.org/docs/latest/extend/plugin-migration-flat-config#backwards-compatibility