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/markdown/examples/markdown_as_child.rs

@@ -1,5 +1,5 @@
 use assets::Assets;
-use gpui::*;
+use gpui::{rgb, App, KeyBinding, Length, StyleRefinement, View, WindowOptions};
 use language::{language_settings::AllLanguageSettings, LanguageRegistry};
 use markdown::{Markdown, MarkdownStyle};
 use node_runtime::NodeRuntime;