Add as_slice to parent (#9871)

# Objective
- Make it possible to write APIs that require a type or homogenous
storage for both `Children` & `Parent` that is agnostic to edge
direction.

## Solution
- Add a way to get the `Entity` from `Parent` as a slice.

---------

Co-authored-by: Alice Cecile <[email protected]>
Co-authored-by: Joseph <[email protected]>

Author: 3 people

crates/bevy_hierarchy/src/components/parent.rs

@@ -23,6 +23,16 @@ impl Parent {
     pub fn get(&self) -> Entity {
         self.0
     }
+
+    /// Gets the parent [`Entity`] as a slice of length 1.
+    ///
+    /// Useful for making APIs that require a type or homogenous storage
+    /// for both [`Children`] & [`Parent`] that is agnostic to edge direction.
+    ///
+    /// [`Children`]: super::children::Children
+    pub fn as_slice(&self) -> &[Entity] {
+        std::slice::from_ref(&self.0)
+    }
 }
 
 // TODO: We need to impl either FromWorld or Default so Parent can be registered as Reflect.