fix(container): rendering scrollable elements with separators

Added corresponding test cases to test the corresponding rendering of
scrollable elements.

examples/elements/scrollable.zig

@@ -73,9 +73,15 @@ pub fn main() !void {
     defer top_box.deinit();
 
     var bottom_box = try App.Container.init(allocator, .{
-        .rectangle = .{ .fill = .blue },
+        .border = .{
+            .sides = .all,
+            .color = .blue,
+        },
         .layout = .{
-            .separator = .{ .enabled = true },
+            .separator = .{
+                .enabled = true,
+                .color = .red,
+            },
             .direction = .vertical,
             .padding = .vertical(1),
         },