Fenced code blocks are supported only at the top level

[ap]

E.g. this works fine:

```sql
SELECT count(*) FROM items WHERE 1=0
```

But if I then go to reply to this:

> ```sql
> SELECT count(*) FROM items WHERE 1=0
> ```

This does not look correct.

Then the fact that this is a fenced code block is not recognised, and instead the tildes are treated as inline code block markers, which in this case causes the entire rest of the file to get mishighlighted.