Add underscore notation [_] for unchecked task list items

[dereuromark]

Summary

• Adds support for underscore notation [_] as an alternative to space [ ] for unchecked task list items
• Related upstream discussion: Allow underscore in task list to denote unchecked item jgm/djot#305 (jgm said "I like the idea")

The underscore notation is useful when:

• Typing on mobile devices where spaces inside brackets can be difficult
• Using editors without monospaced fonts where [ ] may look ambiguous
• The underscore visually resembles an empty checkbox in source

Both notations are fully equivalent and can be mixed within the same list.

Changes

• src/Node/Block/ListItem.php - Explicitly handle _ as unchecked marker (equivalent to space)
• tests/official/task_lists.test - Add official test cases for underscore notation
• tests/TestCase/DjotConverterTest.php - Add unit test
• docs/syntax.md - Document the feature
• docs/enhancements.md - Add to enhancements tracking

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.85%. Comparing base (90010e5) to head (5c16296).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #87   +/-   ##
=========================================
  Coverage     93.85%   93.85%           
- Complexity     2133     2134    +1     
=========================================
  Files            77       77           
  Lines          5708     5708           
=========================================
  Hits           5357     5357           
  Misses          351      351           

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