Introduce Border struct analogous to Shadow

style/src/rule.rs

@@ -1,5 +1,6 @@
 //! Change the appearance of a rule.
-use iced_core::{BorderRadius, Color};
+use crate::core::border;
+use crate::core::Color;
 
 /// The appearance of a rule.
 #[derive(Debug, Clone, Copy)]
@@ -9,7 +10,7 @@ pub struct Appearance {
     /// The width (thickness) of the rule line.
     pub width: u16,
     /// The radius of the line corners.
-    pub radius: BorderRadius,
+    pub radius: border::Radius,
     /// The [`FillMode`] of the rule.
     pub fill_mode: FillMode,
 }