add(border): seperator line options with corresponding code points

examples/container.zig

@@ -63,11 +63,16 @@ pub fn main() !void {
     const element = element_wrapper.element();
 
     var container = try App.Container.init(allocator, .{
-        .border = .{ .separator = .{ .enabled = true } },
+        .border = .{
+            .separator = .{
+                .enabled = true,
+                .line = .double,
+            },
+        },
         .layout = .{
             .gap = 2,
             .padding = .all(5),
-            .direction = .horizontal,
+            .direction = .vertical,
         },
     }, element);
     var box = try App.Container.init(allocator, .{