feat: support async context

### Handling Promise Contexts

```typescript
// Promises in context will be automatically resolved
const result = await evaluate('x + y', {
x: Promise.resolve(5),
y: Promise.resolve(10)
}); // returns 15

// Handling promises returned by async functions
register('fetchData', async () => {
const response = await fetch('https://api.example.com/data');
return response.json();
});

const data = await evaluate('@fetchData()');
```