Update recommended Vitest extension

[NicolasCwy]

📑 Summary

Update extensions.json to use the new VS Code Vitest plugin

Resolves #5321

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 🔖 targeted develop branch

[netlify]

❌ Deploy Preview for mermaid-js failed.

Name	Link
🔨 Latest commit	6f3a260
🔍 Latest deploy log	https://app.netlify.com/sites/mermaid-js/deploys/65dbf25b82e2ae0008bdc35e

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.15%. Comparing base (70c8a3d) to head (6f3a260).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5322   +/-   ##
========================================
  Coverage    79.15%   79.15%           
========================================
  Files          175      175           
  Lines        14514    14514           
  Branches       866      866           
========================================
  Hits         11488    11488           
  Misses        2823     2823           
  Partials       203      203           

Flag	Coverage Δ
e2e	84.72% <ø> (ø)
unit	43.20% <ø> (ø)

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[mermaid-bot]

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