Add Confluence task list renderer for GFM task lists

Convert GFM task list items (- [x] / - [ ]) to Confluence
ac:task-list XML format instead of HTML checkboxes.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

testdata/tasklists-mixed.html

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+<ul>
+<li>[x] task item</li>
+<li>regular item</li>
+<li>[ ] another task item</li>
+</ul>

testdata/tasklists-mixed.md

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+- [x] task item
+- regular item
+- [ ] another task item

testdata/tasklists.html

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+<ac:task-list>
+<ac:task>
+<ac:task-id>1</ac:task-id>
+<ac:task-status>complete</ac:task-status>
+<ac:task-body>#739</ac:task-body>
+</ac:task>
+<ac:task>
+<ac:task-id>2</ac:task-id>
+<ac:task-status>incomplete</ac:task-status>
+<ac:task-body><a href="https://github.com/octo-org/octo-repo/issues/740">https://github.com/octo-org/octo-repo/issues/740</a></ac:task-body>
+</ac:task>
+<ac:task>
+<ac:task-id>3</ac:task-id>
+<ac:task-status>incomplete</ac:task-status>
+<ac:task-body>Add delight to the experience when all tasks are complete :tada:</ac:task-body>
+</ac:task>
+</ac:task-list>

testdata/tasklists.md

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+- [x] #739
+- [ ] https://github.com/octo-org/octo-repo/issues/740
+- [ ] Add delight to the experience when all tasks are complete :tada: