Add world and world_mut methods to RelatedSpawner (#18880)

# Objective

`RelatedSpawnerCommands` offers methods to get the underlying
`Commands`.
`RelatedSpawner` does not expose the inner `World` reference so far.

I currently want to write extension traits for both of them but I need
to duplicate the whole API for the latter because I cannot get it's
`&mut World`.

## Solution

Add methods for immutable and mutable `World` access

crates/bevy_ecs/src/relationship/related_methods.rs

@@ -531,6 +531,16 @@ impl<'w, R: Relationship> RelatedSpawner<'w, R> {
     pub fn target_entity(&self) -> Entity {
         self.target
     }
+
+    /// Returns a reference to the underlying [`World`].
+    pub fn world(&self) -> &World {
+        self.world
+    }
+
+    /// Returns a mutable reference to the underlying [`World`].
+    pub fn world_mut(&mut self) -> &mut World {
+        self.world
+    }
 }
 
 /// Uses commands to spawn related "source" entities with the given [`Relationship`], targeting