feat(fs/unstable): add writeTextFile and writeTextFileSync

[jbronder]

This PR adds writeTextFile and writeTextFileSync APIs to @std/fs package. These functions mirror the Deno.writeTextFile and Deno.writeTextFileSync APIs.

Also, the unstable_write_file.ts is added to the doc checker in this PR.

Towards #6255.

[codecov]

Codecov Report

Attention: Patch coverage is 36.06557% with 39 lines in your changes missing coverage. Please review.

Project coverage is 95.67%. Comparing base (b719b19) to head (9624a11).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
fs/unstable_write_text_file.ts	36.06%	39 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6463      +/-   ##
==========================================
- Coverage   95.77%   95.67%   -0.11%     
==========================================
  Files         574      575       +1     
  Lines       42917    42978      +61     
  Branches     6435     6438       +3     
==========================================
+ Hits        41105    41120      +15     
- Misses       1778     1819      +41     
- Partials       34       39       +5     

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

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[kt3k]

LGTM. Thanks!