Add invalid tag interface

Author: Kirill Nesmeyanov

src/Tag/InvalidTag.php

@@ -4,7 +4,7 @@
 
 namespace TypeLang\PHPDoc\Tag;
 
-final class InvalidTag extends Tag
+final class InvalidTag extends Tag implements InvalidTagInterface
 {
     /**
      * @var non-empty-string

src/Tag/InvalidTagInterface.php

@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace TypeLang\PHPDoc\Tag;
+
+interface InvalidTagInterface extends TagInterface
+{
+    /**
+     * Returns the reason why this tag is invalid.
+     */
+    public function getReason(): \Throwable;
+}