add SystemIdMarker Component to enable filtering for SystemId Entitys (#14584)

# Objective

Enables writing queries like `Query<Entity, With<SystemIdMarker>>` to
filter `Entity`s that are, or are not (with `Without`), `SystemId`s.

## Solution

Simple unit struct `SystemIdMarker` added during
`World::register_boxed_system`; `World::remove_system` already despawns
the entity, removing the marker.

## Testing

No tests, but happy to write some with direction.

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_ecs/src/system/system_registry.rs

@@ -12,6 +12,10 @@ struct RegisteredSystem<I, O> {
     system: BoxedSystem<I, O>,
 }
 
+/// Marker [`Component`](bevy_ecs::component::Component) for identifying [`SystemId`] [`Entity`]s.
+#[derive(Component)]
+pub struct SystemIdMarker;
+
 /// A system that has been removed from the registry.
 /// It contains the system and whether or not it has been initialized.
 ///
@@ -125,10 +129,13 @@ impl World {
     ) -> SystemId<I, O> {
         SystemId {
             entity: self
-                .spawn(RegisteredSystem {
+                .spawn((
+                    RegisteredSystem {
                         initialized: false,
                         system,
-                })
+                    },
+                    SystemIdMarker,
+                ))
                 .id(),
             marker: std::marker::PhantomData,
         }