Added "Invokable Cases" PHPStan extension (#13)

* Added "Invokable Cases" PHPStan extension

This includes a PHPStan extension to add support for `InvokableCases`
so that static analysis tools can understand the callable methods and
their return types.

The extension has been added in a way that will allow multiple extensions
in the future if required, with a single include file that will import
all extensions, or the option to include only specific extensions.

* Added "Invokable Cases" PHPStan extension testing

tests/PHPStan/InvokableCases/Role.php

@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace ArchTech\Enums\Tests\PHPStan\InvokableCases;
+
+use ArchTech\Enums\InvokableCases;
+
+enum Role
+{
+    use InvokableCases;
+
+    case admin;
+    case manager;
+    case staff;
+
+    public static function administrator(): self
+    {
+        return self::admin;
+    }
+
+    public function isManager(): bool
+    {
+        return $this === self::manager;
+    }
+}