This commit has a side-effect that we also now allow mixed lists,
but they have different syntax from the commonmark implementation
and our marked output. For example, without the closing li tags:
Input Bugdown Marked
-------------------------------------
<ul>
- Hello <li>Hello <ul><li>Hello</ul>
+ World <li>World <ul><li>World
+ Again <li>Again <li>Again</ul>
* And <li>And <ul><li>And
* Again <li>Again <li>Again</ul>
</ul>
The bugdown render is in line with what a user in #13447 requests.
Fixes#13477.