feat: delete directory items in parallel

[dundee]

closes #285

[codecov]

Codecov Report

Attention: Patch coverage is 91.52542% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 89.99%. Comparing base (ec0e2bb) to head (794a2fa).

Files	Patch %	Lines
cmd/gdu/app/app.go	0.00%	3 Missing and 1 partial ⚠️
pkg/remove/parallel.go	96.15%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #340      +/-   ##
==========================================
- Coverage   90.62%   89.99%   -0.64%     
==========================================
  Files          34       36       +2     
  Lines        3125     3157      +32     
==========================================
+ Hits         2832     2841       +9     
- Misses        224      245      +21     
- Partials       69       71       +2     

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