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/shadow.rs

@@ -1,4 +1,7 @@
-use gpui::*;
+use gpui::{
+    div, prelude::*, px, rgb, size, App, AppContext, Bounds, ViewContext, WindowBounds,
+    WindowOptions,
+};
 
 struct Shadow {}