add a command to clean the store from empty directories

[amtoine]

this will close #76.

this PR

• adds the fs/dir clean-empty-directories-rec command and it's test tests store-cleaning
• adds the gm clean command and test in tests gm store-cleaning
• cleans the parent of any removed directory after gm remove and test that in tests gm store-cleaning-after-remove

[amtoine]

ok only one more test...

i'll become a Windows expert without ever touching the OS with all this 😆

[amtoine]

daaaaaaaaaaaaaaaaaaaamn 😆
finally 🥺

'twas a missing path expand because Windows was using a relative path 👀