Update default command line handler to not exit the gem unless specified by the user

[gcotelli]

Some applications are forking background processes, and not expecting the main process to terminate. In Launchpad application termination is explicit by calling exitSuccess or exitError.
This change make the GS64 code compatible with Pharo.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (445ccb9) 99.78% compared to head (3b8435d) 99.78%.

Additional details and impacted files

@@                Coverage Diff                 @@
##           release-candidate      #53   +/-   ##
==================================================
  Coverage              99.78%   99.78%           
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  Files                     56       56           
  Lines                   3688     3688           
==================================================
  Hits                    3680     3680           
  Misses                     8        8           

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