feat: support async context #14

// 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()');