Allow extending When attribute

This allows people to extend the When attribute with something like this:

```php
use Attribute;
use Symfony\Component\DependencyInjection\Attribute\When;

#[Attribute(Attribute::TARGET_CLASS | Attribute::TARGET_METHOD | Attribute::TARGET_FUNCTION | Attribute::IS_REPEATABLE)]
final class Exclude extends When
{
    public function __construct()
    {
        parent::__construct('never');
    }
}
```

Then they can use `#[Exclude]` instead of `#[When(env: 'never')]`.

References:
- symfony/symfony#46643
- symfony/symfony#46655

Author: ruudk

CHANGELOG.md

@@ -9,6 +9,7 @@ CHANGELOG
  * Add `enum` env var processor
  * Add `shuffle` env var processor
  * Add `resolve-env` option to `debug:config` command to display actual values of environment variables in dumped configuration
+ * Allow #[When] to be extended
 
 6.1
 ---

Loader/FileLoader.php

@@ -120,7 +120,7 @@ public function registerClasses(Definition $prototype, string $namespace, string
             if (null === $errorMessage && $autoconfigureAttributes && $this->env) {
                 $r = $this->container->getReflectionClass($class);
                 $attribute = null;
-                foreach ($r->getAttributes(When::class) as $attribute) {
+                foreach ($r->getAttributes(When::class, \ReflectionAttribute::IS_INSTANCEOF) as $attribute) {
                     if ($this->env === $attribute->newInstance()->env) {
                         $attribute = null;
                         break;

Loader/PhpFileLoader.php

@@ -101,7 +101,7 @@ private function executeCallback(callable $callback, ContainerConfigurator $cont
         $r = new \ReflectionFunction($callback);
 
         $attribute = null;
-        foreach ($r->getAttributes(When::class) as $attribute) {
+        foreach ($r->getAttributes(When::class, \ReflectionAttribute::IS_INSTANCEOF) as $attribute) {
             if ($this->env === $attribute->newInstance()->env) {
                 $attribute = null;
                 break;

Tests/Fixtures/Prototype/Foo.php

@@ -4,7 +4,7 @@
 
 use Symfony\Component\DependencyInjection\Attribute\When;
 
-#[When(env: 'prod')]
+#[ProductionOnly]
 #[When(env: 'dev')]
 class Foo implements FooInterface, Sub\BarInterface
 {
@@ -16,3 +16,12 @@ public function setFoo(self $foo)
     {
     }
 }
+
+#[\Attribute(\Attribute::TARGET_CLASS | \Attribute::TARGET_METHOD | \Attribute::TARGET_FUNCTION | \Attribute::IS_REPEATABLE)]
+class ProductionOnly extends When
+{
+    public function __construct()
+    {
+        parent::__construct('prod');
+    }
+}