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/hello_world.rs

@@ -1,4 +1,7 @@
-use gpui::*;
+use gpui::{
+    div, prelude::*, px, rgb, size, App, AppContext, Bounds, SharedString, ViewContext,
+    WindowBounds, WindowOptions,
+};
 
 struct HelloWorld {
     text: SharedString,
@@ -11,7 +14,7 @@ impl Render for HelloWorld {
             .flex_col()
             .gap_3()
             .bg(rgb(0x505050))
-            .size(Length::Definite(Pixels(500.0).into()))
+            .size(px(500.0))
             .justify_center()
             .items_center()
             .shadow_lg()