gpui: Add opacity to support transparency of the entire element (#17132)

Release Notes:

- N/A

---

Add this for let GPUI element to support fade in-out animation.

## Platform test

- [x] macOS
- [x] blade `cargo run -p gpui --example opacity --features macos-blade`

## Usage

```rs
div()
    .opacity(0.5)
    .bg(gpui::black())
    .text_color(gpui::black())
    .child("Hello world")
```

This will apply the `opacity` it self and all children to use `opacity`
value to render colors.

## Example

```
cargo run -p gpui --example opacity
cargo run -p gpui --example opacity --features macos-blade
```

<img width="612" alt="image"
src="https://github.com/user-attachments/assets/f1da87ed-31f5-4b55-a023-39e8ee1ba349">

crates/gpui/src/platform/mac/shaders.metal

@@ -385,7 +385,7 @@ fragment float4 polychrome_sprite_fragment(
     color.g = grayscale;
     color.b = grayscale;
   }
-  color.a *= saturate(0.5 - distance);
+  color.a *= sprite.opacity * saturate(0.5 - distance);
   return color;
 }