Add list item attribute support

[dereuromark]

Summary

Adds support for attributes on list items, following the proposed syntax from jgm/djot#262.

Syntax:

- item 1
  {.highlight #id1}
- item 2
  {data-value="test"}

Output:

<ul>
<li class="highlight" id="id1">item 1</li>
<li data-value="test">item 2</li>
</ul>

Works with:

• Unordered lists (-, *)
• Ordered lists (1., a., i.)
• Task lists (- [x], - [ ])

Implementation

• Added parseAttributeStringToArray() method to return attributes without modifying $pendingAttributes
• Modified tryParseList() to detect attribute blocks after list item content
• Attribute lines must be indented at content level (same as continuation text)

Test plan

• 5 new tests for list item attributes
• All 539 existing tests pass
• PHPStan level 8 passes
• Code style passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 86.20690% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.14%. Comparing base (a371cbb) to head (0fae4ed).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Parser/BlockParser.php	86.20%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master       #5      +/-   ##
============================================
- Coverage     92.19%   92.14%   -0.05%     
- Complexity     1377     1392      +15     
============================================
  Files            53       53              
  Lines          3549     3578      +29     
============================================
+ Hits           3272     3297      +25     
- Misses          277      281       +4     

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