Surrounding markdown code blocks with alternate code blocks breaks

[allejo]

It is valid to nest to code blocks in markdown as such:

~~~markdown
```php
<?php

echo someFunction();
```
~~~

You can see how GitHub handles this below:

```php
<?php

echo someFunction();
```

However, this breaks all parsing after these nested code blocks in this plugin.

[vsch]

@allejo, it is an issue with CommonMark parser which need to be upgraded to version 28.1 compatibility.

Previous version treated ~~~ and ``` and interchangeable so the triple tilde is closed by the triple back ticks.

The only workaround for now is to use 4 or more tilde or back ticks on the outer code fence:

~~~~markdown
```php
<?php

echo someFunction();
```
~~~~