When `vitest`, vite config `defineConfig` should come from `vitest/config`

When you select `vitest`, 

```ts
// this import
import { defineConfig } from "vite";

// should switch to 
import { defineConfig } from "vitest/config";
```

If you don't do this & you have 0 import of `vitest` in your project, your `vite.config` will show an error.

cc @paoloricciuti 