mod(container/layout): support gap configuration

examples/container.zig

@@ -23,11 +23,19 @@ pub fn main() !void {
     defer renderer.deinit();
 
     var container = try App.Container.init(allocator, .{
-        .border = .{ .color = .green, .corners = .rounded },
+        .border = .{
+            .color = .green,
+            .corners = .rounded,
+            .separator = .{ .enabled = true },
+        },
+        .layout = .{ .gap = 40 },
     });
     try container.append(try App.Container.init(allocator, .{
         .border = .{ .color = .light_green, .corners = .squared },
     }));
+    try container.append(try App.Container.init(allocator, .{
+        .border = .{ .color = .light_red, .corners = .squared },
+    }));
     defer container.deinit();
 
     // NOTE: should the min-size here be required?