Markdown: improve handling of horizontal rules (#60563)
- support horizontal rules consisting of underscores
- add missing 'breaking' decoration for `horizontalrule` to
fix support for horizontal rules e.g. inside of lists
- add support for horizontal rules to the GitHub flavor (it
seems like an accident that it was missing before)
- slightly rearrange the code to make it easier to
understand (at least for me)
Fixes 6 CommonMark spec tests.
