parse error when there is spaces after `----` in slide mode

In slide mode, we use `----` for branches.

If there are spaces after `----`, the parser could not parse it as branch, but rather see it as a division line in the slide.

Is this behavior on purpose?


Start by locating the slide-mode parser and its handling of lines beginning with `----`; reproduce the difference between a bare marker and one followed by spaces. Confirm the intended behavior with maintainers, then add coverage for the chosen behavior and verify that slide branches and division lines are distinguished correctly.

Scritto dal modello di indicizzazione a partire dal testo della issue.