Extract CodeGen namespace from LintMixinCommand

Replace hardcoded validator class lists with a declarative #[Mixin]
attribute and extract the mixin generation logic into a reusable
CodeGen namespace under src-dev/CodeGen/.

The new MixinGenerator discovers prefix definitions and filtering
rules by scanning #[Mixin] attributes on the target namespace's
classes, removing the need for hardcoded configuration. It supports
configurable interface types (Builder for __callStatic, Chain for
__call) with custom suffixes, return types, and root extends.

This is the first step toward extracting the code generation into a
standalone package that can map __call/__callStatic to any namespace,
possibly for Respect/StringFormatter and any kind of project in the
future.

src-dev/CodeGen/InterfaceConfig.php

@@ -0,0 +1,28 @@
+<?php
+
+/*
+ * SPDX-License-Identifier: MIT
+ * SPDX-FileCopyrightText: (c) Respect Project Contributors
+ * SPDX-FileContributor: Alexandre Gomes Gaigalas <alganet@gmail.com>
+ */
+
+declare(strict_types=1);
+
+namespace Respect\Dev\CodeGen;
+
+final readonly class InterfaceConfig
+{
+    /**
+     * @param array<string> $rootExtends
+     * @param array<string> $rootUses
+     */
+    public function __construct(
+        public string $suffix,
+        public string $returnType,
+        public bool $static = false,
+        public array $rootExtends = [],
+        public string|null $rootComment = null,
+        public array $rootUses = [],
+    ) {
+    }
+}