Add support for custom numbering of ordered list using [@\d+]

org mode supports [1] setting the value attribute [2] of ordered list items to
change the numbering for the current and following items. Let's do the same. As
the attribute has no meaning for other types of lists [2] we'll just not
support it for those cases [3].

[1] https://orgmode.org/manual/Plain-Lists.html#Plain-Lists
[2] https://developer.mozilla.org/en-US/docs/Web/HTML/Element/li#attributes
[3]
Org mode seems to instead set the id attribute for e.g. unordered lists
starting with `[@\d+]\s` - but I don't really see the value in that and will
skip that for now.

org/testdata/lists.html

@@ -195,3 +195,14 @@ ordered descriptive
 <p>unordered 2</p>
 </li>
 </ul>
+<ol>
+<li value="2">
+<p>use `[@n]` to change the value of list items</p>
+</li>
+<li class="unchecked">
+<p>foobar</p>
+</li>
+<li value="10" class="checked">
+<p>that even works in combination with list statuses (`[ ]`)</p>
+</li>
+</ol>

org/testdata/lists.org

@@ -68,3 +68,8 @@ some list termination tests
 2. ordered descriptive :: 2
 - unordered 1
 - unordered 2
+
+
+1. [@2] use `[@n]` to change the value of list items
+2. [ ] foobar
+3. [@10] [X] that even works in combination with list statuses (`[ ]`)

org/testdata/lists.pretty_org

@@ -68,3 +68,8 @@ some list termination tests
 2. ordered descriptive :: 2
 - unordered 1
 - unordered 2
+
+
+1. [@2] use `[@n]` to change the value of list items
+2. [ ] foobar
+3. [@10] [X] that even works in combination with list statuses (`[ ]`)