Adds expect extend

src/Support/Extendable.php

@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Pest\Support;
+
+use Closure;
+
+final class Extendable
+{
+    /**
+     * The extendable class.
+     *
+     * @var string
+     */
+    private $extendableClass;
+
+    /**
+     * Creates a new extendable instance.
+     */
+    public function __construct(string $extendableClass)
+    {
+        $this->extendableClass = $extendableClass;
+    }
+
+    /**
+     * Register a custom extend.
+     */
+    public function extend(string $name, Closure $extend): void
+    {
+        $this->extendableClass::extend($name, $extend);
+    }
+}