Add helpers to send Events from World (#5355)

# Objective

- With access to `World`, it's not obvious how to send an event.
- This is especially useful if you are writing a `Command` that needs to send an `Event`.
- `Events` are a first-class construct in bevy, even though they are just `Resources` under the hood. Their methods should be discoverable.

## Solution

- Provide a simple helpers to send events through `Res<Events<T>>`.
---

## Changelog

> `send_event`, `send_default_event`, and `send_event_batch` methods added to `World`.

Author: aevyrie

crates/bevy_ecs/src/world/mod.rs

@@ -1156,6 +1156,30 @@ impl World {
         result
     }
 
+    /// Sends an [`Event`](crate::event::Event).
+    #[inline]
+    pub fn send_event<E: crate::event::Event>(&mut self, event: E) {
+        self.send_event_batch(std::iter::once(event));
+    }
+
+    /// Sends the default value of the [`Event`](crate::event::Event) of type `E`.
+    #[inline]
+    pub fn send_default_event<E: crate::event::Event + Default>(&mut self) {
+        self.send_event_batch(std::iter::once(E::default()));
+    }
+
+    /// Sends a batch of [`Event`](crate::event::Event)s from an iterator.
+    #[inline]
+    pub fn send_event_batch<E: crate::event::Event>(&mut self, events: impl Iterator<Item = E>) {
+        match self.get_resource_mut::<crate::event::Events<E>>() {
+            Some(mut events_resource) => events_resource.extend(events),
+            None => bevy_utils::tracing::error!(
+                    "Unable to send event `{}`\n\tEvent must be added to the app with `add_event()`\n\thttps://docs.rs/bevy/*/bevy/app/struct.App.html#method.add_event ",
+                    std::any::type_name::<E>()
+                ),
+        }
+    }
+
     /// # Safety
     /// `component_id` must be assigned to a component of type `R`
     #[inline]