feat(fs/unstable): add symlink and symlinkSync

[jbronder]

This PR adds the symlink and symlinkSync APIs to @std/fs, which are functions intended to mirror Deno.symlink and Deno.symlinkSync, respectively. The PR also adds the associated SymlinkOptions type.

Towards #6255.

[codecov]

Codecov Report

Attention: Patch coverage is 55.26316% with 17 lines in your changes missing coverage. Please review.

Project coverage is 96.33%. Comparing base (5d41054) to head (0d53a6d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
fs/unstable_symlink.ts	55.26%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6352      +/-   ##
==========================================
- Coverage   96.37%   96.33%   -0.05%     
==========================================
  Files         550      551       +1     
  Lines       41880    41918      +38     
  Branches     6351     6353       +2     
==========================================
+ Hits        40363    40380      +17     
- Misses       1477     1498      +21     
  Partials       40       40              

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[kt3k]

LGTM. Nice work! Thanks