Add observer to Trigger (#15066)

# Objective

- Fixes  #15061

## Solution

- Added `observer` to `Trigger`, which returns the entity observing the
triggered event.

## Testing

- CI passed locally.

crates/bevy_ecs/src/observer/mod.rs

@@ -56,11 +56,37 @@ impl<'w, E, B: Bundle> Trigger<'w, E, B> {
  Ptr::from(&self.event)
  }
 
- /// Returns the entity that triggered the observer, could be [`Entity::PLACEHOLDER`].
+ /// Returns the [`Entity`] that triggered the observer, could be [`Entity::PLACEHOLDER`].
  pub fn entity(&self) -> Entity {
  self.trigger.entity
  }
 
+ /// Returns the [`Entity`] that observed the triggered event.
+ /// This allows you to despawn the observer, ceasing observation.
+ ///
+ /// # Examples
+ ///
+ /// ```rust
+ /// # use bevy_ecs::prelude::{Commands, Trigger};
+ /// #
+ /// # struct MyEvent {
+ /// # done: bool,
+ /// # }
+ /// #
+ /// /// Handle `MyEvent` and if it is done, stop observation.
+ /// fn my_observer(trigger: Trigger<MyEvent>, mut commands: Commands) {
+ /// if trigger.event().done {
+ /// commands.entity(trigger.observer()).despawn();
+ /// return;
+ /// }
+ ///
+ /// // ...
+ /// }
+ /// ```
+ pub fn observer(&self) -> Entity {
+ self.trigger.observer
+ }
+
  /// Enables or disables event propagation, allowing the same event to trigger observers on a chain of different entities.
  ///
  /// The path an event will propagate along is specified by its associated [`Traversal`] component. By default, events