fix: don't extend root's parent class for facade stubs in helper file (#1766)

Since #1674 (commit 09623f2), the generated IDE helper file adds an
`extends` declaration for Macroable classes to expose inherited methods.
However, the check to skip facades only excluded classes in the
`\Illuminate\Support\Facades` namespace, so third-party facades (e.g.
`Spatie\Menu\Laravel\Facades\Menu`) would incorrectly extend the
facade root's parent class.

This caused the generated stub to conflict with the real facade class:

    namespace Spatie\Menu\Laravel\Facades {
        class Menu extends \Spatie\Menu\Menu {  // wrong!

The fix checks whether the extends class IS a Facade subclass (using
`is_subclass_of`) rather than comparing namespace strings. This
correctly excludes ALL facade classes (both Laravel and third-party)
while still allowing non-facade Macroable classes to extend their
parent.

Fixes #1724

src/Alias.php

@@ -187,7 +187,7 @@ class Alias
     public function shouldExtendParentClass()
     {
         return $this->parentClass
-            && $this->getExtendsNamespace() !== '\\Illuminate\\Support\\Facades';
+            && !is_subclass_of($this->extends, \Illuminate\Support\Facades\Facade::class);
     }
 
     /**