Hello, I've been loving using GPUI! Recently I noticed that calling
`.with_animiation()` would give an `AnimationElement<E>` which did not
allow `.child()` to be called on it. It was a quick fix, I hope it's a
quick easy merge but let me know if you'd like anything changed :)

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

Co-authored-by: Lukas Wirth <lukas@zed.dev>