[eslint-plugin-react-compiler] fix recommended config usage with defineConfig

[kasmacioma]

Summary

Recently ESLint released defineConfig utility to work with flat configs. Now when using recommended config from eslint-plugin-react-compiler, a typing error appears because the severity for the react-compiler/react-compiler rule in the .d.ts file is of type string rather than the more specific "error". Here is a simple reproduction.

How did you test this change?

Just recreated the recommended config in TypeScript playground with the suggested changes

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