feat(fs/unstable): add chmod and chmodSync

[jbronder]

This PR adds the chmod and chmodSync APIs to @std/fs, which are functions intended to mirror Deno.chmod and Deno.chmodSync, respectively.

Towards #6255.

[codecov]

Codecov Report

Attention: Patch coverage is 51.72414% with 14 lines in your changes missing coverage. Please review.

Project coverage is 96.39%. Comparing base (2604b37) to head (3bd2293).

Files with missing lines	Patch %	Lines
fs/unstable_chmod.ts	51.72%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6343      +/-   ##
==========================================
- Coverage   96.43%   96.39%   -0.04%     
==========================================
  Files         547      548       +1     
  Lines       41812    41841      +29     
  Branches     6347     6346       -1     
==========================================
+ Hits        40321    40334      +13     
- Misses       1450     1466      +16     
  Partials       41       41              

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