Save original literal in ThematicBreak on parsing

[robinst]

Fixes #295.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.75%. Comparing base (6385869) to head (bd40c08).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #309   +/-   ##
=========================================
  Coverage     94.75%   94.75%           
  Complexity      254      254           
=========================================
  Files           131      131           
  Lines          4135     4141    +6     
  Branches        604      604           
=========================================
+ Hits           3918     3924    +6     
  Misses          108      108           
  Partials        109      109           

Files	Coverage Δ
...a/org/commonmark/internal/ThematicBreakParser.java	96.66% <100.00%> (+0.37%)	⬆️
...c/main/java/org/commonmark/node/ThematicBreak.java	100.00% <100.00%> (ø)