feat: add fsx file exist and write file funcs

[emmanuelgautier]

No description provided.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.00%. Comparing base (0208c3b) to head (a30e2e6).

Files with missing lines	Patch %	Lines
fsx/fsx.go	90.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #37      +/-   ##
==========================================
+ Coverage   57.50%   64.00%   +6.50%     
==========================================
  Files           2        2              
  Lines          80      100      +20     
==========================================
+ Hits           46       64      +18     
- Misses         28       30       +2     
  Partials        6        6              

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