feat(fs/unstable): add readLink and readLinkSync

[jbronder]

This PR adds the readLink and readLinkSync APIs to @std/fs, which are functions intended to mirror Deno.readLink and Deno.readLinkSync, 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 96.25%. Comparing base (61ee836) to head (4e26303).
Report is 2 commits behind head on main.

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6373      +/-   ##
==========================================
- Coverage   96.29%   96.25%   -0.04%     
==========================================
  Files         554      555       +1     
  Lines       42002    42031      +29     
  Branches     6365     6367       +2     
==========================================
+ Hits        40445    40459      +14     
- Misses       1517     1532      +15     
  Partials       40       40              

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[kt3k]

LGTM. Nice!