[9.x] Add sole() to Enumerable contract. (#37066)

* Added sole() to Enumerable contract.

* Update Enumerable.php

Co-authored-by: Taylor Otwell <taylor@laravel.com>

Author: ash-jc-allen

Enumerable.php

@@ -808,6 +808,19 @@ public function slice($offset, $length = null);
      */
     public function split($numberOfGroups);
 
+    /**
+     * Get the first item in the collection, but only if exactly one item exists. Otherwise, throw an exception.
+     *
+     * @param  mixed  $key
+     * @param  mixed  $operator
+     * @param  mixed  $value
+     * @return mixed
+     *
+     * @throws \Illuminate\Collections\ItemNotFoundException
+     * @throws \Illuminate\Collections\MultipleItemsFoundException
+     */
+    public function sole($key = null, $operator = null, $value = null);
+
     /**
      * Chunk the collection into chunks of the given size.
      *