Parent -> ChildOf (#17427)

Fixes #17412

## Objective

`Parent` uses the "has a X" naming convention. There is increasing
sentiment that we should use the "is a X" naming convention for
relationships (following #17398). This leaves `Children` as-is because
there is prevailing sentiment that `Children` is clearer than `ParentOf`
in many cases (especially when treating it like a collection).

This renames `Parent` to `ChildOf`.

This is just the implementation PR. To discuss the path forward, do so
in #17412.

## Migration Guide

- The `Parent` component has been renamed to `ChildOf`.

benches/benches/bevy_ecs/entity_cloning.rs

@@ -3,7 +3,7 @@ use core::hint::black_box;
 use benches::bench;
 use bevy_ecs::bundle::Bundle;
 use bevy_ecs::component::ComponentCloneHandler;
-use bevy_ecs::hierarchy::Parent;
+use bevy_ecs::hierarchy::ChildOf;
 use bevy_ecs::reflect::AppTypeRegistry;
 use bevy_ecs::{component::Component, world::World};
 use bevy_math::Mat4;
@@ -142,7 +142,7 @@ fn bench_clone_hierarchy<B: Bundle + Default + GetTypeRegistration>(
 
         for parent_id in current_hierarchy_level {
             for _ in 0..children {
-                let child_id = world.spawn((B::default(), Parent(parent_id))).id();
+                let child_id = world.spawn((B::default(), ChildOf(parent_id))).id();
                 hierarchy_level.push(child_id);
             }
         }

benches/benches/bevy_ecs/observers/propagation.rs

@@ -65,7 +65,7 @@ pub fn event_propagation(criterion: &mut Criterion) {
 struct TestEvent<const N: usize> {}
 
 impl<const N: usize> Event for TestEvent<N> {
-    type Traversal = &'static Parent;
+    type Traversal = &'static ChildOf;
     const AUTO_PROPAGATE: bool = true;
 }
 

crates/bevy_app/src/app.rs

@@ -107,7 +107,7 @@ impl Default for App {
         {
             app.init_resource::<AppTypeRegistry>();
             app.register_type::<Name>();
-            app.register_type::<Parent>();
+            app.register_type::<ChildOf>();
             app.register_type::<Children>();
         }
 

crates/bevy_ecs/src/entity/clone_entities.rs

@@ -12,7 +12,7 @@ use crate::{
     bundle::Bundle,
     component::{Component, ComponentCloneHandler, ComponentId, ComponentInfo, Components},
     entity::Entity,
-    hierarchy::{Children, Parent},
+    hierarchy::{ChildOf, Children},
     query::DebugCheckedUnwrap,
     world::{DeferredWorld, World},
 };
@@ -631,11 +631,11 @@ impl<'w> EntityCloneBuilder<'w> {
     /// Sets the option to add cloned entity as a child to the parent entity.
     pub fn as_child(&mut self, as_child: bool) -> &mut Self {
         if as_child {
-            self.override_component_clone_handler::<Parent>(ComponentCloneHandler::custom_handler(
+            self.override_component_clone_handler::<ChildOf>(ComponentCloneHandler::custom_handler(
                 component_clone_parent,
             ))
         } else {
-            self.remove_component_clone_handler_override::<Parent>()
+            self.remove_component_clone_handler_override::<ChildOf>()
         }
     }
 
@@ -694,18 +694,21 @@ fn component_clone_children(world: &mut DeferredWorld, ctx: &mut ComponentCloneC
             .with_source_and_target(*child, child_clone);
         world.commands().queue(move |world: &mut World| {
             clone_entity.clone_entity(world);
-            world.entity_mut(child_clone).insert(Parent(parent));
+            world.entity_mut(child_clone).insert(ChildOf(parent));
         });
     }
 }
 
-/// Clone handler for the [`Parent`] component. Allows to add clone as a child to the parent entity.
+/// Clone handler for the [`ChildOf`] component. Allows to add clone as a child to the parent entity.
 fn component_clone_parent(world: &mut DeferredWorld, ctx: &mut ComponentCloneCtx) {
     let parent = ctx
-        .read_source_component::<Parent>()
+        .read_source_component::<ChildOf>()
         .map(|p| p.0)
-        .expect("Source entity must have Parent component");
-    world.commands().entity(ctx.target()).insert(Parent(parent));
+        .expect("Source entity must have a ChildOf component");
+    world
+        .commands()
+        .entity(ctx.target())
+        .insert(ChildOf(parent));
 }
 
 #[cfg(test)]