TextContentRenderer: Fix overriding of core node rendering

[robinst]

It's correct for HTML rendering and there's a test there as well. This adds a test for text content and fixes the behavior. Fixes #166.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.02%. Comparing base (4634a00) to head (cebb010).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #346   +/-   ##
=========================================
  Coverage     95.02%   95.02%           
  Complexity      252      252           
=========================================
  Files           137      137           
  Lines          4380     4380           
  Branches        620      620           
=========================================
  Hits           4162     4162           
  Misses          116      116           
  Partials        102      102           

Files with missing lines	Coverage Δ
.../commonmark/renderer/text/TextContentRenderer.java	93.02% <100.00%> (ø)