implement Debug for RichText (emilk#5596)

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* Derive Debug for RichText
* [x] I have followed the instructions in the PR template

crates/egui/src/widget_text.rs

@@ -22,7 +22,7 @@ use crate::{
 /// RichText::new("colored").color(Color32::RED);
 /// RichText::new("Large and underlined").size(20.0).underline();
 /// ```
-#[derive(Clone, Default, PartialEq)]
+#[derive(Debug, Clone, Default, PartialEq)]
 pub struct RichText {
     text: String,
     size: Option<f32>,