export `defineConfig` function

### Is your feature request related to a problem? Please describe.

as an alternative to `type CodegenConfig`

### Describe the solution you'd like

```ts
import { defineConfig } from '@graphql-codegen/cli'

export default defineConfig({
// your config
})
```

### Describe alternatives you've considered

_No response_

### Is your feature request related to a problem? Please describe.

_No response_