Add widget context

[StarArawn]

Widget context should exist local to the tree where it is spawned. API example:

// parent.rs
let context_entity = api.use_context::<T>(current_widget);
commands.entity(context_entity).spawn(MyContext);

// child.rs
let context_entity = api.use_context::<T>(current_widget);
let Ok(context) = context_query.get(context_entity) else {
    return;
}