Add support for dashed borders to GPUI (#27139)

Features:

* Scales dash spacing with border width.
* Laying out dashes around rounded corners.
* Varying border widths with rounded corners - now uses an ellipse for the inner edge of the border.
* When there are no rounded corners, each straight border is laid out separately, so that the dashes to meet at the corners.
* All sides of each dash are antialiased.

![image](https://github.com/user-attachments/assets/b3789a98-a5be-4f97-9736-c4e59615afe6)

![image](https://github.com/user-attachments/assets/739bdc57-4580-42c8-bfc3-6e287411a408)

Release Notes:

- N/A

---------

Co-authored-by: Michael Sloan <michael@zed.dev>
Co-authored-by: Ben <ben@zed.dev>

crates/gpui/src/styled.rs

@@ -1,7 +1,8 @@
 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, UnderlineStyle, WhiteSpace,
+    self as gpui, px, relative, rems, AbsoluteLength, AlignItems, BorderStyle, CursorStyle,
+    DefiniteLength, Fill, FlexDirection, FlexWrap, Font, FontStyle, FontWeight, Hsla,
+    JustifyContent, Length, SharedString, StrikethroughStyle, StyleRefinement, TextOverflow,
+    UnderlineStyle, WhiteSpace,
 };
 use crate::{TextAlign, TextStyleRefinement};
 pub use gpui_macros::{
@@ -361,6 +362,12 @@ pub trait Styled: Sized {
         self
     }
 
+    /// Sets the border style of the element.
+    fn border_dashed(mut self) -> Self {
+        self.style().border_style = Some(BorderStyle::Dashed);
+        self
+    }
+
     /// Returns a mutable reference to the text style that has been configured on this element.
     fn text_style(&mut self) -> &mut Option<TextStyleRefinement> {
         let style: &mut StyleRefinement = self.style();