Add a trait for commands that run for a given Entity (bevyengine#7015)

# Objective

Resolve bevyengine#6156.

The most common type of command is one that runs for a single entity. Built-in commands like this can be ergonomically added to the command queue using the `EntityCommands` struct. However, adding custom entity commands to the queue is quite cumbersome. You must first spawn an entity, store its ID in a local, then construct a command using that ID and add it to the queue. This prevents method chaining, which is the main benefit of using `EntityCommands`.

### Example (before)

```rust
struct MyCustomCommand(Entity);

impl Command for MyCustomCommand { ... }

let id = commands.spawn((...)).id();
commmands.add(MyCustomCommand(id));
```

## Solution

Add the `EntityCommand` trait, which allows directly adding per-entity commands to the `EntityCommands` struct.

### Example (after)

```rust
struct MyCustomCommand;

impl EntityCommand for MyCustomCommand { ... }

commands.spawn((...)).add(MyCustomCommand);
```
---

## Changelog

- Added the trait `EntityCommand`. This is a counterpart of `Command` for types that execute code for a single entity.

## Future Work

If we feel its necessary, we can simplify built-in commands (such as `Despawn`) to use this trait.

crates/bevy_ecs/src/system/commands/mod.rs

@@ -528,6 +528,85 @@ impl<'w, 's> Commands<'w, 's> {
     }
 }
 
+/// A [`Command`] which gets executed for a given [`Entity`].
+///
+/// # Examples
+///
+/// ```
+/// # use std::collections::HashSet;
+/// # use bevy_ecs::prelude::*;
+/// use bevy_ecs::system::EntityCommand;
+/// #
+/// # #[derive(Component, PartialEq)]
+/// # struct Name(String);
+/// # impl Name {
+/// #   fn new(s: String) -> Self { Name(s) }
+/// #   fn as_str(&self) -> &str { &self.0 }
+/// # }
+///
+/// #[derive(Resource, Default)]
+/// struct Counter(i64);
+///
+/// /// A `Command` which names an entity based on a global counter.
+/// struct CountName;
+///
+/// impl EntityCommand for CountName {
+///     fn write(self, id: Entity, world: &mut World) {
+///         // Get the current value of the counter, and increment it for next time.
+///         let mut counter = world.resource_mut::<Counter>();
+///         let i = counter.0;
+///         counter.0 += 1;
+///
+///         // Name the entity after the value of the counter.
+///         world.entity_mut(id).insert(Name::new(format!("Entity #{i}")));
+///     }
+/// }
+///
+/// // App creation boilerplate omitted...
+/// # let mut world = World::new();
+/// # world.init_resource::<Counter>();
+/// #
+/// # let mut setup_stage = SystemStage::single_threaded().with_system(setup);
+/// # let mut assert_stage = SystemStage::single_threaded().with_system(assert_names);
+/// #
+/// # setup_stage.run(&mut world);
+/// # assert_stage.run(&mut world);
+///
+/// fn setup(mut commands: Commands) {
+///     commands.spawn_empty().add(CountName);
+///     commands.spawn_empty().add(CountName);
+/// }
+///
+/// fn assert_names(named: Query<&Name>) {
+///     // We use a HashSet because we do not care about the order.
+///     let names: HashSet<_> = named.iter().map(Name::as_str).collect();
+///     assert_eq!(names, HashSet::from_iter(["Entity #0", "Entity #1"]));
+/// }
+/// ```
+pub trait EntityCommand: Send + 'static {
+    fn write(self, id: Entity, world: &mut World);
+    /// Returns a [`Command`] which executes this [`EntityCommand`] for the given [`Entity`].
+    fn with_entity(self, id: Entity) -> WithEntity<Self>
+    where
+        Self: Sized,
+    {
+        WithEntity { cmd: self, id }
+    }
+}
+
+/// Turns an [`EntityCommand`] type into a [`Command`] type.
+pub struct WithEntity<C: EntityCommand> {
+    cmd: C,
+    id: Entity,
+}
+
+impl<C: EntityCommand> Command for WithEntity<C> {
+    #[inline]
+    fn write(self, world: &mut World) {
+        self.cmd.write(self.id, world);
+    }
+}
+
 /// A list of commands that will be run to modify an [entity](crate::entity).
 pub struct EntityCommands<'w, 's, 'a> {
     entity: Entity,
@@ -690,6 +769,27 @@ impl<'w, 's, 'a> EntityCommands<'w, 's, 'a> {
         });
     }
 
+    /// Pushes an [`EntityCommand`] to the queue, which will get executed for the current [`Entity`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use bevy_ecs::prelude::*;
+    /// # fn my_system(mut commands: Commands) {
+    /// commands
+    ///     .spawn_empty()
+    ///     // Closures with this signature implement `EntityCommand`.
+    ///     .add(|id: Entity, world: &mut World| {
+    ///         println!("Executed an EntityCommand for {id:?}");
+    ///     });
+    /// # }
+    /// # bevy_ecs::system::assert_is_system(my_system);
+    /// ```
+    pub fn add<C: EntityCommand>(&mut self, command: C) -> &mut Self {
+        self.commands.add(command.with_entity(self.entity));
+        self
+    }
+
     /// Logs the components of the entity at the info level.
     ///
     /// # Panics
@@ -716,6 +816,15 @@ where
     }
 }
 
+impl<F> EntityCommand for F
+where
+    F: FnOnce(Entity, &mut World) + Send + 'static,
+{
+    fn write(self, id: Entity, world: &mut World) {
+        self(id, world);
+    }
+}
+
 #[derive(Debug)]
 pub struct Spawn<T> {
     pub bundle: T,