Add methods to add single entity relationships (#18038)

# Objective

Closes #17572

## Solution

Add the `add_one_related` methods to `EntityCommands` and
`EntityWorldMut`.

## Testing

Clippy

---

## Showcase

The `EntityWorldMut` and `FilteredResourcesMut` now include the
`add_one_related` method if you just want to relate 2 entities.

Author: Brezak

crates/bevy_ecs/src/relationship/related_methods.rs

@@ -21,7 +21,9 @@ impl<'w> EntityWorldMut<'w> {
         self
     }
 
-    /// Relates the given entities to this entity with the relation `R`
+    /// Relates the given entities to this entity with the relation `R`.
+    ///
+    /// See [`add_one_related`](Self::add_one_related) if you want relate only one entity.
     pub fn add_related<R: Relationship>(&mut self, related: &[Entity]) -> &mut Self {
         let id = self.id();
         self.world_scope(|world| {
@@ -32,6 +34,13 @@ impl<'w> EntityWorldMut<'w> {
         self
     }
 
+    /// Relates the given entity to this with the relation `R`.
+    ///
+    /// See [`add_related`](Self::add_related) if you want to relate more than one entity.
+    pub fn add_one_related<R: Relationship>(&mut self, entity: Entity) -> &mut Self {
+        self.add_related::<R>(&[entity])
+    }
+
     /// Despawns entities that relate to this one via the given [`RelationshipTarget`].
     /// This entity will not be despawned.
     pub fn despawn_related<S: RelationshipTarget>(&mut self) -> &mut Self {
@@ -108,7 +117,9 @@ impl<'a> EntityCommands<'a> {
         self
     }
 
-    /// Relates the given entities to this entity with the relation `R`
+    /// Relates the given entities to this entity with the relation `R`.
+    ///
+    /// See [`add_one_related`](Self::add_one_related) if you want relate only one entity.
     pub fn add_related<R: Relationship>(&mut self, related: &[Entity]) -> &mut Self {
         let id = self.id();
         let related = related.to_vec();
@@ -120,6 +131,13 @@ impl<'a> EntityCommands<'a> {
         self
     }
 
+    /// Relates the given entity to this with the relation `R`.
+    ///
+    /// See [`add_related`](Self::add_related) if you want to relate more than one entity.
+    pub fn add_one_related<R: Relationship>(&mut self, entity: Entity) -> &mut Self {
+        self.add_related::<R>(&[entity])
+    }
+
     /// Despawns entities that relate to this one via the given [`RelationshipTarget`].
     /// This entity will not be despawned.
     pub fn despawn_related<S: RelationshipTarget>(&mut self) -> &mut Self {