Invalid `flatConfigs` types

[nbouvrette]

With the recent official support of ESLint TypeScript config (ref), when trying to setup the example config in the README file, I get the following typings issue:

'importPlugin.flatConfigs' is possibly 'undefined'.

I created temporary d.ts file as a workaround for now which is not ideal:

declare module 'eslint-plugin-import' {
  import type { Linter } from 'eslint'

  export const flatConfigs: {
    recommended: Linter.Config
    typescript: Linter.Config
    [key: string]: Linter.Config | undefined
  }
}

This also fix another type issue for importPlugin.flatConfigs.recommended as it currently uses any. Ideally all the types should be fixed to avoid TypeScript config errors.