This revises the fix to #8150 (and the test case) and closes#9187.
HTML in the (invalid) form:
<ul>
<li>L1</li>
<ul>
<li>L1.1</li>
</ul>
</ul>
is treated by browsers like
<ul>
<li>L1
<ul>
<li>L1.1</li>
</ul>
</li>
</ul>
not
<ul>
<li>L1
<li><ul>
<li>L1.1</li>
</ul>
</li>
</ul>
as pandoc previously assumed.
This change will give a similar treatment to
<ul>
<li>L1</li>
<p>foobar</p>
</ul>
which also seems to match browser behavior.
The HTML standard requires all list items to be marked with a `<li>`
tag, but some tools fail to do so for sublists. The reader now accepts
these unwrapped lists as sublists.
Closes: #8150