gpui: Remove use of use gpui::* in examples (#22311)

This PR removes the use of `use gpui::*` in the GPUI examples, as this
is not how consumers should be importing GPUI.

Release Notes:

- N/A

crates/gpui/examples/opacity.rs

@@ -1,6 +1,10 @@
 use std::{fs, path::PathBuf, time::Duration};
 
-use gpui::*;
+use anyhow::Result;
+use gpui::{
+    div, hsla, img, point, prelude::*, px, rgb, size, svg, App, AppContext, AssetSource, Bounds,
+    BoxShadow, ClickEvent, SharedString, Task, Timer, ViewContext, WindowBounds, WindowOptions,
+};
 
 struct Assets {
     base: PathBuf,
@@ -76,7 +80,7 @@ impl Render for HelloWorld {
             .flex()
             .flex_row()
             .size_full()
-            .bg(rgb(0xE0E0E0))
+            .bg(rgb(0xe0e0e0))
             .text_xl()
             .child(
                 div()