New option lineSeparator for MarkdownRenderer.Builder

[robinst]

Fixes #442

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.70%. Comparing base (76af766) to head (e45a480).

Files with missing lines	Patch %	Lines
...g/commonmark/renderer/markdown/MarkdownWriter.java	73.33%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #446      +/-   ##
============================================
- Coverage     94.76%   94.70%   -0.07%     
  Complexity      423      423              
============================================
  Files           156      156              
  Lines          5313     5327      +14     
  Branches        788      788              
============================================
+ Hits           5035     5045      +10     
- Misses          144      148       +4     
  Partials        134      134              

Files with missing lines	Coverage Δ
...commonmark/renderer/markdown/MarkdownRenderer.java	100.00% <100.00%> (ø)
...g/commonmark/renderer/markdown/MarkdownWriter.java	92.15% <73.33%> (-3.50%)	⬇️

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