add -u flag to cp, mv command

[WindSoilder]

Description

Closes: #7853

I found that I want this feature too...

So I take over it, sorry for that @VincenzoCarlino

User-Facing Changes

Tests + Formatting

After Submitting

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@8695b57). Click here to learn what that means.
The diff coverage is 95.34%.

❗ Current head e92a0d8 differs from pull request most recent head 8962d11. Consider uploading reports for the commit 8962d11 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9214   +/-   ##
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  Coverage        ?   68.93%           
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  Files           ?      635           
  Lines           ?   102081           
  Branches        ?        0           
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  Hits            ?    70374           
  Misses          ?    31707           
  Partials        ?        0           

Impacted Files	Coverage Δ
crates/nu-command/src/filesystem/mv.rs	81.96% <92.30%> (ø)
crates/nu-command/src/filesystem/util.rs	77.88% <93.33%> (ø)
crates/nu-command/src/filesystem/cp.rs	69.74% <100.00%> (ø)

[amtoine]

this looks like it's working just fine to me 👌

i'm just wondering if we should add two tests to make sure the copy works indeed when the source is newer than the destination 🤔
currently we only test when source is older 😮

[WindSoilder]

Thanks for your review! I've added one more action in the test case to match relative scenario

[amtoine]

thanks @WindSoilder 😊