add(element/radio-button): RadioButton Element implementation

This can be used to visualize the values of `bool`'s, which is relevant
when creating form's based on `struct`'s automatically.

src/app.zig

@@ -416,6 +416,7 @@ pub fn App(comptime E: type) type {
         pub const Button = element.Button(Event, Queue);
         pub const Input = element.Input(Event, Queue);
         pub const Progress = element.Progress(Event, Queue);
+        pub const RadioButton = element.RadioButton(Event);
         pub const Scrollable = element.Scrollable(Event);
         pub const Queue = queue.Queue(Event, 256);
     };