feat(fs/unstable): add link and linkSync

[jbronder]

This PR adds the link and linkSync APIs to @std/fs, which are functions intended to mirror Deno.link and Deno.linkSync, 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 96.28%. Comparing base (f30741c) to head (b991d31).
Report is 2 commits behind head on main.

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6369      +/-   ##
==========================================
- Coverage   96.31%   96.28%   -0.04%     
==========================================
  Files         553      554       +1     
  Lines       41973    42002      +29     
  Branches     6360     6363       +3     
==========================================
+ Hits        40426    40441      +15     
- Misses       1507     1521      +14     
  Partials       40       40              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[kt3k]

LGTM. Nice!