Force psalm to accept toArray as pure

See previous commit.

Psalm isn't actually wrong here, but we are willing to lie to psalm
since the impurity is just an internal cache.

src/Enum.php

@@ -146,6 +146,7 @@ public static function values()
      * Returns all possible values as an array
      *
      * @psalm-pure
+     * @psalm-suppress ImpureStaticProperty
      *
      * @psalm-return array<string, mixed>
      * @return array Constant name in key, constant value in value