feat(element/alignment): alignment Element implementation

You can now align a `Container` using the Alignment `Element` similar to
how you make a `Container` scrollable. For usage details please see the
example and the corresponding tests.

src/app.zig

@@ -397,6 +397,7 @@ pub fn App(comptime E: type) type {
         pub const Event = mergeTaggedUnions(event.SystemEvent, E);
         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 Exec = element.Exec(Event, Queue);
         pub const Queue = queue.Queue(Event, 256);