Broken default export with Vite 8 / Rolldown

## Bug: Broken default export with Vite 8 / Rolldown

### Description

After upgrading to **Vite 8**, importing `react-countup` breaks with the following warning:

```
Warning: React.jsx: type is invalid -- expected a string (for built-in components) or a class/function (for composite components) but got: object.
```

### Root cause

Vite 8 switched from esbuild/Rollup to **Rolldown** as its bundler. Rolldown changed the CJS interop behavior: when `module.exports.__esModule === true`, the default import now resolves to `module.exports.default` instead of `module.exports` itself.

Because `react-countup` ships as CJS with `__esModule: true`, the default import returns the whole module object instead of the component function:

```js
import CountUp from 'react-countup';
console.log(CountUp);
// → { __esModule: true, default: ƒ CountUp, useCountUp: ƒ }
// Expected: ƒ CountUp
```

### Steps to reproduce

1. Use Vite 8 (Rolldown)
2. Import `react-countup` as default import
3. Render `` — React throws the invalid type warning

### Current workaround

Adding `legacy.inconsistentCjsInterop: true` in `vite.config.js` restores the old behavior, but this option is marked as deprecated.

### Expected fix

The package should either:
- Ship an **ESM build** (`"type": "module"` or a proper `exports` field in `package.json`)
- Or ensure the CJS bundle is compatible with the new Rolldown interop behavior (i.e. not set `__esModule: true` unless the default export is correct)

### References
- [Vite 8 Migration Guide – CJS Interop](https://vite.dev/guide/migration)
- [Rolldown CJS Interop Docs](https://rolldown.rs)

### Environment

| | |
|---|---|
| react-countup | latest |
| Vite | 8.x |
| Rolldown | (bundled with Vite 8) |
| React | 18 / 19 |