feat(fs/unstable): add readDir

[jbronder]

This PR adds the readDir API to @std/fs, which is a function intended to mirror Deno.readDir.

Towards #6255.

[codecov]

Codecov Report

Attention: Patch coverage is 28.20513% with 28 lines in your changes missing coverage. Please review.

Project coverage is 96.27%. Comparing base (9b86f0f) to head (509ff08).

Files with missing lines	Patch %	Lines
fs/unstable_read_dir.ts	33.33%	20 Missing ⚠️
fs/_to_dir_entry.ts	11.11%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6338      +/-   ##
==========================================
- Coverage   96.36%   96.27%   -0.09%     
==========================================
  Files         547      549       +2     
  Lines       41731    41770      +39     
  Branches     6325     6325              
==========================================
+ Hits        40212    40214       +2     
- Misses       1478     1515      +37     
  Partials       41       41              

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