Print violations to stdout, everything else to stderr

[micheleorselli]

Here the output is divided between STDOUT and STDERR, in particular:

• the list of violations is printed on stdout
• everything else on stderr

This has several benefits

• we can remove the $onlyError logic scattered in the code
• the output now can be piped to other commands or in a file and in case of json/gitlab format you end up having a valid json

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.42%. Comparing base (d7f99f7) to head (04d3888).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #484      +/-   ##
============================================
- Coverage     95.47%   95.42%   -0.06%     
+ Complexity      640      621      -19     
============================================
  Files            73       73              
  Lines          1724     1704      -20     
============================================
- Hits           1646     1626      -20     
  Misses           78       78              

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[AlessandroMinoccheri]

LGTM 👍