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/elements/div.rs

@@ -1662,7 +1662,7 @@ impl Interactivity {
         window: &mut Window,
         cx: &mut App,
     ) {
-        use crate::TextAlign;
+        use crate::{BorderStyle, TextAlign};
 
         if global_id.is_some()
             && (style.debug || style.debug_below || cx.has_global::<crate::DebugBelow>())
@@ -1753,6 +1753,7 @@ impl Interactivity {
                                 },
                             },
                             crate::red(),
+                            BorderStyle::default(),
                         ))
                     }
                 }