fix: use global namespace for ValueError (#5)

Fixes exception:
```
Error: Class "vlw\ValueError" not found
```

Reviewed-on: https://codeberg.org/vlw/php-xenum/pulls/5

src/xEnum.php

@@ -23,7 +23,7 @@
         // Throw a ValueError if Enum name is not found
         public static function fromName(?string $name): static {
             $case = self::tryFromName($name);
-            return $case ? $case : throw new ValueError("'{$name}' is not a valid case for enum " . self::class);
+            return $case ? $case : throw new \ValueError("'{$name}' is not a valid case for enum " . self::class);
         }
 
         // Return array of enum names