add: Allow 0 in group limits for unlimited parallel tasks

[Nukesor]

Fixes #500

[codecov]

Codecov Report

Attention: 24 lines in your changes are missing coverage. Please review.

Comparison is base (42af45a) 79.67% compared to head (6b845cb) 80.15%.
Report is 13 commits behind head on development.

Files	Patch %	Lines
pueue/src/bin/pueue.rs	20.00%	24 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #501      +/-   ##
===============================================
+ Coverage        79.67%   80.15%   +0.47%     
===============================================
  Files               77       77              
  Lines             5564     5673     +109     
===============================================
+ Hits              4433     4547     +114     
+ Misses            1131     1126       -5     

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