feat: add useRiveList hook (#90)

## Summary
- Adds `useRiveList` hook for managing list properties, aligned with
rive-react API
- Adds `useViewModelInstance` hook that works with `RiveFile`,
`ViewModel`, and `RiveViewRef`
- Adds `required` option to `useViewModelInstance` for Error Boundary
support
- Adds MenuList example demonstrating list manipulation

### useRiveList API
```tsx
const { length, getInstanceAt, addInstance, addInstanceAt, removeInstance, removeInstanceAt, swap, error } = useRiveList('items', viewModelInstance);
```

### useViewModelInstance API
```tsx
// From RiveFile (get default instance) - recommended
const { riveFile } = useRiveFile(require('./animation.riv'));
const instance = useViewModelInstance(riveFile);
<RiveView file={riveFile} dataBind={instance} ... />

// From RiveViewRef (get auto-bound instance)
const { riveViewRef, setHybridRef } = useRive();
const instance = useViewModelInstance(riveViewRef);

// With required: true (throws if null, use with Error Boundary)
const instance = useViewModelInstance(riveFile, { required: true });
// TypeScript knows instance is non-null
```

### Example Pattern
```tsx
function MenuList({ file }: { file: RiveFile }) {
  const instance = useViewModelInstance(file, { required: true });
  return <MenuListContent file={file} instance={instance} />;
}
```

## Test plan
- [x] MenuListExample uses `useRiveList` and `useViewModelInstance(file,
{ required: true })`

Author: mfazekas