feat: make dateFormat function more efficient

[nagypeterjob]

PR fo #80

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 🎉

Comparison is base (8a998e5) 97.15% compared to head (5e2f6d2) 97.18%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   97.15%   97.18%   +0.03%     
==========================================
  Files          12       12              
  Lines         632      639       +7     
==========================================
+ Hits          614      621       +7     
  Misses         18       18              

Impacted Files	Coverage Δ
Sources/Puppy/FileRotationLogger.swift	96.40% <100.00%> (+0.13%)	⬆️
Sources/Puppy/LogFormatter.swift	100.00% <100.00%> (ø)

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[sushichop]

Thanks. This PR makes sense👍
In addition, it's cool that the performance measurement has been added in tests🙂