[3.4-dev] Fix async flag for TL_JAVASCRIPT

Currently the `async` flag doesn’t work correctly:

```php
$GLOBALS['TL_JAVASCRIPT'][] = 'script.js|async';
// Output: <script src="script.js"></script>

$GLOBALS['TL_JAVASCRIPT'][] = 'script.js|static|async';
// Output: <script src="assets/js/e057c6f76856.js"></script>
```

After the fix:

```php
$GLOBALS['TL_JAVASCRIPT'][] = 'script.js|async';
// Output: <script src="script.js" async></script>

$GLOBALS['TL_JAVASCRIPT'][] = 'script.js|static|async';
// Output: <script src="assets/js/9e6682604751.js" async></script>
```

Related: #7172