gpui: Update Shadow Example (#22434)

This PR adds a more comprehensive shadow example to gpui:

![CleanShot 2024-12-26 at 13 54
05@2x](https://github.com/user-attachments/assets/452fb8a1-d294-4b56-b0e0-f4e4ca6c29d4)

This is prep to work on the following issues:

- Shadows with a blur of 0 do not render. Expected: A shadow with sharp
edges (#22433)
- Spread does not correctly conform to the shape the shadow is cast from

Release Notes:

- N/A