Add Decimal alias for StringDecimal with usage example

- Introduced `Decimal` as a readonly alias for `StringDecimal`.
- Updated `Usage` examples to demonstrate `Decimal` usage and behavior.
- Minor annotation update in `StringCountryCode` to ignore mutation testing.

Author: georgedeploy

src/String/Alias/Decimal.php

@@ -0,0 +1,14 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpTypedValues\String\Alias;
+
+use PhpTypedValues\String\MariaDb\StringDecimal;
+
+/**
+ * @psalm-immutable
+ */
+readonly class Decimal extends StringDecimal
+{
+}

src/String/StringCountryCode.php

@@ -69,6 +69,8 @@ public function value(): string
     /**
      * ISO 3166-1 alpha-2 codes used for validation.
      *
+     * @pest-mutate-ignore
+     *
      * @return list<non-empty-string>
      */
     private static function listAllowed(): array

src/Usage/String.php

@@ -1,6 +1,7 @@
 <?php
 
 use PhpTypedValues\String\Alias\CountryCode;
+use PhpTypedValues\String\Alias\Decimal;
 use PhpTypedValues\String\Alias\Email;
 use PhpTypedValues\String\Alias\JsonStr;
 use PhpTypedValues\String\Alias\NonBlankStr;
@@ -74,6 +75,7 @@
 }
 
 // MariaDb Decimal (usage and try*) and toFloat strictness
+echo Decimal::fromString('3.14')->toString() . \PHP_EOL;
 echo StringDecimal::fromString('3.14')->toString() . \PHP_EOL;
 // tryFromString branch
 $dec = StringDecimal::tryFromString('1.5');