A small flexbox/grid UI layer for the Gizmo engine.
gizmo-ui builds on [taffy] for layout and integrates with the Gizmo ECS.
UI elements are entities carrying components such as [Style] (layout),
[Node] (computed geometry), [BackgroundColor] and [Interaction].
Spawn them via the [NodeBundle] and [ButtonBundle] bundles.
Add [UiPlugin] to an App to register the components and run the layout
and interaction systems each frame. Common types are re-exported from the
[prelude] module.