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/styled.rs

@@ -547,6 +547,12 @@ pub trait Styled: Sized {
         self
     }
 
+    /// Set opacity on this element and its children.
+    fn opacity(mut self, opacity: f32) -> Self {
+        self.style().opacity = Some(opacity);
+        self
+    }
+
     /// Draw a debug border around this element.
     #[cfg(debug_assertions)]
     fn debug(mut self) -> Self {