feat: add ORDER BY statement Enum (#41)

Importable with:
```php
use vlw\MySQL\Order
```
To be used with the `MySQL->order()` method, for example:
```php
$db->for("table")->order(["column" => Order::ASC])->select("*");
```

Reviewed-on: https://codeberg.org/vlw/php-mysql/pulls/41

src/Order.php

@@ -0,0 +1,8 @@
+<?php
+
+	namespace vlw\MySQL;
+
+	enum Order: string {
+		case ASC  = "ASC";
+		case DESC = "DESC";
+	}