fs: fix cpSync to handle non-ASCII characters

[StefanStojanovic]

This PR fixes the problem described in the linked issue and adds a regression test for it.

Fixes: #61878

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.77%. Comparing base (da5efc4) to head (1a71ee2).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_file.cc	66.66%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61950      +/-   ##
==========================================
+ Coverage   88.84%   89.77%   +0.93%     
==========================================
  Files         674      674              
  Lines      204957   205609     +652     
  Branches    39309    39410     +101     
==========================================
+ Hits       182087   184589    +2502     
+ Misses      15088    13264    -1824     
+ Partials     7782     7756      -26     

Files with missing lines	Coverage Δ
src/node_file.cc	75.30% <66.66%> (+0.03%)	⬆️

... and 179 files with indirect coverage changes

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