add(container/rectangle): add content creation

examples/container.zig

@@ -27,7 +27,7 @@ pub fn main() !void {
         .border = .{
             .color = .blue,
             .sides = .{
-                .top = false,
+                .left = false,
                 .bottom = false,
             },
             .corners = .rounded,
@@ -37,6 +37,7 @@ pub fn main() !void {
     });
     try container.append(try App.Container.init(allocator, .{
         .border = .{ .color = .light_blue, .corners = .squared },
+        .rectangle = .{ .fill = .blue },
     }));
     try container.append(try App.Container.init(allocator, .{
         .border = .{ .color = .light_blue, .corners = .squared },