feat(element/button): add builtin Element implementation for buttons

src/app.zig

@@ -417,8 +417,9 @@ pub fn App(comptime E: type) type {
         pub const Container = @import("container.zig").Container(Event);
         pub const Element = element.Element(Event);
         pub const Alignment = element.Alignment(Event);
-        pub const Scrollable = element.Scrollable(Event);
+        pub const Button = element.Button(Event, Queue);
         pub const Input = element.Input(Event, Queue);
+        pub const Scrollable = element.Scrollable(Event);
         pub const Queue = queue.Queue(Event, 256);
     };
 }