Support cleanup functions for refs

[NotWoods]

Describe the feature you'd love to see
Similar to the new React 19 feature, it would be great to be able to use cleanup functions with callback refs.

<input
  ref={(ref) => {
    // ref created

    // REQUESTED: return a cleanup function to reset
    // the ref when element is removed from DOM.
    return () => {
      // ref cleanup
    };
  }}
/>

This would make it easier to connect Preact components to DOM APIs like MutationObserver, which currently need some hooks/effects. Using object refs and effects is more error-prone as it requires syncing up whatever conditional rendering logic you have between your returned elements and effects.

Additional context (optional)
https://react.dev/blog/2024/04/25/react-19#cleanup-functions-for-refs