`ExtractComponent` refactor

Previously, SyncComponentPlugin/ExtractComponentPlugin would despawn the render entity if the synced component was removed. This removes all the derived components too. In 0.19, the render entity is no longer despawned and only the Target components of SyncComponent trait are removed.

SyncComponent is a subtrait of ExtractComponent and you must implement it to clean up extracted and derived components.

impl SyncComponent for MyComponent {
    type Target = (Self, OtherDerivedComponents);
}

impl ExtractComponent for MyComponent {
    type QueryData = ();
    type QueryFilter = ();
    type Out = Self;

    fn extract_component(
        item: QueryItem<'_, '_, Self::QueryData>,
    ) -> Option<Self::Out> {
        Some(*item)
    }
}

You can also specify the sync target (default to Self) using extract_component_sync_target attribute in derive macros.

#[derive(Component, ExtractComponent)]
#[extract_component_sync_target((Self, OtherDerivedComponents))]
struct MyComponent;

Both SyncComponent and ExtractComponent have also gotten an optional marker type that can be used to bypass orphan rules, see the docs for details.