Underscores incorrectly handled in links

```
[http://localhost:8000/login/random_token](http://localhost:8000/login/random_token)
```

If a link contains two underscores _ it's considered as some em text

Reproduce the example link containing underscores in the Djot parser and inspect the link and emphasis parsing paths. Done means the URL remains a link and its underscores are not interpreted as emphasis, with a regression test covering the reported input.

Written by the indexing model from the issue text.