feat: Add stub files for validation rules

stubs/fields/array_of.stub.php

@@ -0,0 +1,12 @@
+<?php
+/** @generate-class-entries */
+
+namespace Attributes\Validation\Fields {
+   #[Attribute(Attribute::TARGET_PROPERTY|Attribute::TARGET_PARAMETER)]
+   class ArrayOf implements Field {
+      /**
+       * @param class-string|string ...$types
+       */
+      public function __construct(public string ...$types) {}
+   }
+}

stubs/fields/length.stub.php

@@ -0,0 +1,9 @@
+<?php
+/** @generate-class-entries */
+
+namespace Attributes\Validation\Fields {
+   #[Attribute(Attribute::TARGET_PROPERTY|Attribute::TARGET_PARAMETER)]
+   class Length implements Field {
+      public function __construct(public ?int $min = null, public ?int $max = null) {}
+   }
+}

stubs/rules/custom.stub.php

@@ -0,0 +1,12 @@
+<?php
+/** @generate-class-entries */
+
+namespace Attributes\Validation\Rules {
+   interface Custom {
+      /**
+       * Validates the value and returns true if valid, false otherwise.
+       * Can also throw ValidationException for more detailed error messages.
+       */
+      public function validate(mixed $value): bool;
+   }
+}