Add callback to memorize leave function.

[AntonGreger]

Description

This PR memoizes the leave callback in the useEcho hook using useCallback to prevent unnecessary re-renders and improve performance when the function is passed to child components or used in dependency arrays.

Example: Avoiding Unnecessary Re-renders

With the previous implementation, using leave in a useEffect dependency array would cause the effect to run on every render because a new function reference was created each time:

function MyComponent() {
  const { leave } = useEcho('my-channel', 'event', callback, []);
  
  useEffect(() => {
    // Cleanup handler that uses leave
    return () => {
      leave(); // This effect runs on EVERY render because leave is a new function each time
    };
  }, [leave]); // ❌ leave reference changes every render
  
  return <div>My Component</div>;
}

With this PR, the memoized leave function maintains a stable reference (unless dependencies change), preventing unnecessary effect re-runs:

function MyComponent() {
  const { leave } = useEcho('my-channel', 'event', callback, []);
  
  useEffect(() => {
    // Cleanup handler that uses leave
    return () => {
      leave(); // Effect only runs when dependencies actually change
    };
  }, [leave]); // ✅ leave reference is stable
  
  return <div>My Component</div>;
}

[joetannenbaum]

Thanks!