gpui: Add underline style method (#24784)

Release Notes:

- N/A

Add a shorter method to apply underline style.

https://tailwindcss.com/docs/text-decoration-line#underling-text

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/gpui/src/styled.rs

@@ -1,7 +1,7 @@
 use crate::{
     self as gpui, px, relative, rems, AbsoluteLength, AlignItems, CursorStyle, DefiniteLength,
     Fill, FlexDirection, FlexWrap, Font, FontStyle, FontWeight, Hsla, JustifyContent, Length,
-    SharedString, StrikethroughStyle, StyleRefinement, TextOverflow, WhiteSpace,
+    SharedString, StrikethroughStyle, StyleRefinement, TextOverflow, UnderlineStyle, WhiteSpace,
 };
 use crate::{TextAlign, TextStyleRefinement};
 pub use gpui_macros::{
@@ -486,6 +486,17 @@ pub trait Styled: Sized {
         self
     }
 
+    /// Sets the text decoration to underline.
+    /// [Docs](https://tailwindcss.com/docs/text-decoration-line#underling-text)
+    fn underline(mut self) -> Self {
+        let style = self.text_style().get_or_insert_with(Default::default);
+        style.underline = Some(UnderlineStyle {
+            thickness: px(1.),
+            ..Default::default()
+        });
+        self
+    }
+
     /// Sets the decoration of the text to have a line through it.
     /// [Docs](https://tailwindcss.com/docs/text-decoration#setting-the-text-decoration)
     fn line_through(mut self) -> Self {