fix: markdown parser doesn't recognize softline breaks

[LucasXu0]

closes #912

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.

Project coverage is 72.02%. Comparing base (35f0f66) to head (8c62169).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...down/decoder/parser/markdown_paragraph_parser.dart	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #933      +/-   ##
==========================================
+ Coverage   72.00%   72.02%   +0.01%     
==========================================
  Files         318      318              
  Lines       15023    15036      +13     
==========================================
+ Hits        10818    10829      +11     
- Misses       4205     4207       +2     

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