fix: reject info-string lines as code-fence closers

[icyJoseph]

The markdown-code-fence-validity accepts any matching number of ``` backticks as a valid terminator for a fence.

That is a problem for this kind of fenced content:

```
content
``` filename="foo.rs"
```

The current parser ends the block at the ```filename="foo.rs" - but that's not following spec. See what Github renders:

content
``` filename="foo.rs"

One block. Another example, similar to the unit test - though the unit test uses 4 backticks.

In GitHub-flavored markdown you can attach attributes to a fence:
```md filename="example.md"
The closing fence matches the opening fence length, with no info.

[dacharyc]

Another good catch - thanks for the PR, @icyJoseph !