feat(fs/unstable): add truncate and truncateSync

[jbronder]

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

Towards #6255.

[codecov]

Codecov Report

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

Project coverage is 96.01%. Comparing base (431ed58) to head (6175d7b).
Report is 3 commits behind head on main.

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6416      +/-   ##
==========================================
- Coverage   96.03%   96.01%   -0.03%     
==========================================
  Files         561      562       +1     
  Lines       42395    42424      +29     
  Branches     6384     6387       +3     
==========================================
+ Hits        40714    40733      +19     
- Misses       1641     1652      +11     
+ Partials       40       39       -1     

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[kt3k]

Mostly looks good to me.

We started adding unstable fs APIs to doc checker (_tools/check_docs.ts) since #6400. Can you also add this file to that check?

[kt3k]

LGTM. Thanks!