feat(fs/unstable): add mkdir and mkdirSync

[jbronder]

This PR adds mkdir and mkdirSync APIs to the @std/fs package which are intended to mirror Deno.mkdir and Deno.mkdirSync functions.

Towards #6255.

[codecov]

Codecov Report

Attention: Patch coverage is 54.83871% with 14 lines in your changes missing coverage. Please review.

Project coverage is 96.04%. Comparing base (360851f) to head (88feec9).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
fs/unstable_mkdir.ts	54.83%	14 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6436   +/-   ##
=======================================
  Coverage   96.03%   96.04%           
=======================================
  Files         562      563    +1     
  Lines       42418    42449   +31     
  Branches     6387     6390    +3     
=======================================
+ Hits        40738    40770   +32     
- Misses       1641     1646    +5     
+ Partials       39       33    -6     

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[kt3k]

LGTM. Nice!