Separate soft break mode from significantNewlines parsing

[dereuromark]

Summary

• Adds optional softBreakMode parameter to DjotConverter constructor and withSignificantNewlines() factory
• When explicitly provided, overrides automatic behavior
• Backwards compatible: significantNewlines alone still implies <br> soft breaks

Motivation

The significantNewlines mode couples two unrelated concerns:

1. Parsing relaxation - blocks can interrupt paragraphs, nested lists don't need blank lines
2. Rendering behavior - soft breaks become visible <br> tags

This change allows using relaxed parsing without <br> tags:

// Relaxed parsing with normal soft breaks (no <br>)
$converter = DjotConverter::withSignificantNewlines(
    softBreakMode: SoftBreakMode::Space,
);

// Or via constructor
$converter = new DjotConverter(
    significantNewlines: true,
    softBreakMode: SoftBreakMode::Space,
);

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.94%. Comparing base (79a8a34) to head (49bd3b6).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #126      +/-   ##
============================================
+ Coverage     93.92%   93.94%   +0.01%     
- Complexity     2383     2384       +1     
============================================
  Files            81       81              
  Lines          6306     6310       +4     
============================================
+ Hits           5923     5928       +5     
+ Misses          383      382       -1     

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