[4.1] [BUG?] Support for nested ternary statement

There are occasions where nested ternary statements are useful. Before this commit, the following line did not produce expected results:

```
{{ $foo or ($bar or 'foobar') }}
```

Which would have been compiled to:

```php
<?php echo isset($foo) ? $foo : ($bar or 'foobar'); ?>
```

Now it correctly parses the nested statement and compiled to:

```php
<?php echo isset($foo) ? $foo : (isset($bar) ? $bar : 'foobar'); ?>
```