feat: add "flat/all" config

[yeonjuan]

Checklist

• Addresses an existing open issue: fixes feature_request(preset): ...eslintPluginHtml.configs["flat/all"] #436

Description

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.49%. Comparing base (e88504c) to head (c7abce7).

Files with missing lines	Patch %	Lines
packages/eslint-plugin/lib/index.js	33.33%	2 Missing ⚠️

❌ Your patch check has failed because the patch coverage (75.00%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #446      +/-   ##
==========================================
- Coverage   98.56%   98.49%   -0.07%     
==========================================
  Files          84       85       +1     
  Lines        2791     2799       +8     
  Branches      778      778              
==========================================
+ Hits         2751     2757       +6     
- Misses         40       42       +2     

Flag	Coverage Δ
unittest	98.49% <75.00%> (-0.07%)	⬇️

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Files with missing lines	Coverage Δ
packages/eslint-plugin/lib/configs/all.js	100.00% <100.00%> (ø)
packages/eslint-plugin/lib/index.js	69.23% <33.33%> (-10.77%)	⬇️

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