Add support for list item checkboxes (e.g. [X])

see https://orgmode.org/manual/Checkboxes.html
We're deviating from Org mode regarding the assigned css classes but the chosen
classes feel better than (on, off, trans) and it's not like the export matches
1:1 otherwise.

org/testdata/lists.html

@@ -1,5 +1,5 @@
 <ul>
-<li>
+<li class="unchecked">
 <p>
 unordered list item 1
 </p>
@@ -9,22 +9,22 @@ unordered list item 1
 unordered list item 2 - with <code>inline</code> <em>markup</em>
 </p>
 <ol>
-<li>
+<li class="indeterminate">
 <p>
 ordered sublist item 1
 </p>
 <ol>
-<li>
+<li class="checked">
 <p>
 ordered sublist item 1
 </p>
 </li>
-<li>
+<li class="unchecked">
 <p>
 ordered sublist item 2
 </p>
 </li>
-<li>
+<li class="checked">
 <p>
 ordered sublist item 3
 </p>
@@ -38,7 +38,7 @@ ordered sublist item 2
 </li>
 </ol>
 </li>
-<li>
+<li class="checked">
 <p>
 unordered list item 3 - and a <a href="https://example.com">link</a>
 and some lines of text
@@ -95,20 +95,20 @@ that spans multiple lines
 descriptive lists
 </p>
 <dl>
-<dt>
+<dt class="unchecked">
 term<dd>
 <p>
 details
 continued details
 </p>
 <dd>
-<dt>
+<dt class="unchecked">
 ?<dd>
 <p>
 details without a term
 </p>
 <dd>
-<dt>
+<dt class="checked">
 term<dd>
 <p>
 details on a new line