MySqlDriver driver knows about subqueries now. (#265)

As MySQL/MariaDB clearly does support subqueries.

This enables proper building of queries like...
```
$table('table_a')->where('table_a.column_one IN', $table('table_b')->select('value'))
```

... without executing the inner query first.

See https://dev.mysql.com/doc/refman/8.0/en/subqueries.html
and https://mariadb.com/kb/en/subqueries/

src/Database/Drivers/MySqlDriver.php

@@ -204,6 +204,6 @@ public function isSupported(string $item): bool
 		// - http://bugs.mysql.com/bug.php?id=31188
 		// - http://bugs.mysql.com/bug.php?id=35819
 		// and more.
-		return $item === self::SUPPORT_SELECT_UNGROUPED_COLUMNS || $item === self::SUPPORT_MULTI_COLUMN_AS_OR_COND;
+		return $item === self::SUPPORT_SELECT_UNGROUPED_COLUMNS || $item === self::SUPPORT_MULTI_COLUMN_AS_OR_COND || $item === self::SUPPORT_SUBSELECT;
 	}
 }