Split definition list content by blank lines into separate dd elements

[dereuromark]

Summary

• Blank lines within definition content now create separate <dd> elements
• Previously this only happened with multiple terms (: Term1, : Term2)
• Now single terms also split by blank lines for consistency
• Terms with no content still create empty <dd>

Before

<dt>Djot</dt>
<dd>
<p>First paragraph</p>
<p>Second paragraph</p>
</dd>

After

<dt>Djot</dt>
<dd><p>First paragraph</p></dd>
<dd><p>Second paragraph</p></dd>

Spec deviation

This deviates from the djot spec, which keeps all content in one <dd>. The spec doesn't explicitly address this, so it's an implementation choice. The change provides more semantic HTML structure where each distinct definition gets its own <dd> element.

Test plan

• Added tests for blank-line splitting behavior
• Added test for empty <dd> creation
• All 922 unit tests pass
• 1 official test suite deviation (expected - intentional spec difference)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.58%. Comparing base (a073415) to head (6a74a14).

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #48      +/-   ##
============================================
- Coverage     93.63%   93.58%   -0.05%     
+ Complexity     1913     1900      -13     
============================================
  Files            74       74              
  Lines          5136     5115      -21     
============================================
- Hits           4809     4787      -22     
- Misses          327      328       +1     

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[dereuromark]

Superseded by new approach - using explicit : + marker for multiple dd elements instead of blank line splitting.
This helps to not violate the current specs.
#49