core: add .kotlin folder to ignore files

[Khoyo]

See https://youtrack.jetbrains.com/issue/KT-58223/Kotlin-Gradle-plugin-shouldnt-store-data-in-project-cache-directory#focus=Comments-27-8370225.0-0

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 36.94%. Comparing base (df44230) to head (2be79cc).
Report is 6 commits behind head on dev.

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Additional details and impacted files

@@             Coverage Diff              @@
##                dev    #8892      +/-   ##
============================================
- Coverage     37.02%   36.94%   -0.08%     
  Complexity     2211     2211              
============================================
  Files          1255     1256       +1     
  Lines        114227   114423     +196     
  Branches       3182     3182              
============================================
- Hits          42294    42278      -16     
- Misses        70042    70254     +212     
  Partials       1891     1891              

Flag	Coverage Δ
core	74.84% <ø> (ø)
editoast	72.39% <ø> (-0.06%)	⬇️
front	14.90% <ø> (+<0.01%)	⬆️
gateway	2.20% <ø> (ø)
osrdyne	2.60% <ø> (-0.11%)	⬇️
railjson_generator	87.49% <ø> (ø)
tests	86.37% <ø> (ø)

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