HTML writer: Unwrap "wrapper" divs.

Some of the readers (e.g. djot) add "wrapper" divs to hold attributes
for elements that have no slot for attributes in the pandoc AST. With
this change, the HTML reader "unwraps" these wrappers so that the
attributes go on the intended elements.

Closes #11014.

test/command/11014.md

@@ -0,0 +1,9 @@
+```
+% pandoc -f djot -t html
+{.foo}
+- bar
+^D
+<ul class="foo">
+<li>bar</li>
+</ul>
+```