irmin-pack: add classification for volume files

[metanivek]

As pointed out in #2197, we need classification for volumes. So this PR does that. 😎

[codecov-commenter]

Codecov Report

Merging #2201 (cbea0b2) into main (fc9d3cb) will increase coverage by 0.03%.
The diff coverage is 92.30%.

❗ Current head cbea0b2 differs from pull request most recent head b91dfe5. Consider uploading reports for the commit b91dfe5 to get more accurate results

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@@            Coverage Diff             @@
##             main    #2201      +/-   ##
==========================================
+ Coverage   67.97%   68.01%   +0.03%     
==========================================
  Files         135      135              
  Lines       16301    16302       +1     
==========================================
+ Hits        11081    11088       +7     
+ Misses       5220     5214       -6     

Impacted Files	Coverage Δ
src/irmin-pack/unix/file_manager.ml	83.86% <60.00%> (ø)
src/irmin-pack/layout.ml	83.63% <100.00%> (+0.30%)	⬆️
src/irmin-fs/unix/irmin_fs_unix.ml	68.38% <0.00%> (+3.87%)	⬆️

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[clecat]

LGTM, thx a lot