feat(fs/unstable): add fs.stat

[kt3k]

This PR adds stat API, which mostly mirrors Deno.stat, to @std/fs.

towards #6255

[codecov]

Codecov Report

Attention: Patch coverage is 47.96748% with 64 lines in your changes missing coverage. Please review.

Project coverage is 96.35%. Comparing base (283cf34) to head (306bb5c).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
fs/_to_file_info.ts	7.14%	26 Missing ⚠️
fs/_map_error.ts	38.88%	11 Missing ⚠️
fs/unstable_errors.js	71.05%	11 Missing ⚠️
fs/unstable_stat.ts	52.63%	9 Missing ⚠️
fs/_utils.ts	65.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6258      +/-   ##
==========================================
- Coverage   96.49%   96.35%   -0.14%     
==========================================
  Files         536      541       +5     
  Lines       41179    41303     +124     
  Branches     6175     6179       +4     
==========================================
+ Hits        39734    39798      +64     
- Misses       1403     1463      +60     
  Partials       42       42              

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