Fix SemanticSpanExtension samp and var rendering

[dereuromark]

Summary

• render samp and var semantic span attributes as <samp> and <var>
• remove those processed attributes from the outer span before rendering
• add extension regression coverage for standalone and combined samp/var

Testing

• vendor/bin/phpunit tests/TestCase/Extension/SemanticSpanExtensionTest.php tests/TestCase/Converter/HtmlToDjotTest.php
• vendor/bin/phpstan analyse src --no-progress
• vendor/bin/phpunit

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.98%. Comparing base (345a47b) to head (8a3e5e6).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #170   +/-   ##
=========================================
  Coverage     93.98%   93.98%           
- Complexity     3135     3139    +4     
=========================================
  Files            93       93           
  Lines          8009     8017    +8     
=========================================
+ Hits           7527     7535    +8     
  Misses          482      482           

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