chore: configure PHPStan Level 6 and resolve strict-type warnings

src/Rules/AriaRole.php

@@ -8,6 +8,7 @@ use YakNet\AccessibilityConsole\Core\WCAGStandard;
 
 class AriaRole extends AbstractRule
 {
+    /** @var array<int, string> */
     private static array $validRoles = [
         'alert', 'alertdialog', 'application', 'article', 'banner', 'button', 'cell', 'checkbox',
         'columnheader', 'combobox', 'complementary', 'contentinfo', 'definition', 'dialog', 'directory',

src/Rules/ColorContrast.php

@@ -92,6 +92,9 @@ class ColorContrast extends AbstractRule
         return null;
     }
 
+    /**
+     * @return array{0: int, 1: int, 2: int}|null
+     */
     private function hexToRgb(string $hex): ?array
     {
         $hex = ltrim($hex, '#');