build: enable preserveModules to main bundle so consumers can use tree shaking

Rollup is not configure to `preserveModules` for the `src/index.ts` entry, resulting in one ESM and one JS file for the entire library.

We'd like to allow consumers to import individual components and be able to tree shake the files that are not used.

```js
import { Button } from 'react95';
```

Should only include files necessary for the Button to render.