Fix source spans of blocks with lazy continuation lines

[robinst]

Fixes #337. Thanks to @sylvainheraud for most of the test cases!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.01%. Comparing base (6bebfe2) to head (7be4204).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #339   +/-   ##
=========================================
  Coverage     95.01%   95.01%           
  Complexity      254      254           
=========================================
  Files           136      136           
  Lines          4353     4353           
  Branches        618      618           
=========================================
  Hits           4136     4136           
  Misses          116      116           
  Partials        101      101           

Files with missing lines	Coverage Δ
...n/java/org/commonmark/internal/DocumentParser.java	98.18% <100.00%> (ø)