fix: deduplicate Vitest excludes entries

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: blockVitest should dedupe exclude entries #2082
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Also fixes a yet-to-be-hit-by-users bug where only having "node_modules" there results in [, "node_modules"].

🎁

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.82%. Comparing base (f24a019) to head (ff2cc53).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2083   +/-   ##
=======================================
  Coverage   99.82%   99.82%           
=======================================
  Files         122      122           
  Lines        5079     5081    +2     
  Branches      341      341           
=======================================
+ Hits         5070     5072    +2     
  Misses          9        9           

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[github-actions]

🎉 This is included in version v2.21.3 🎉

The release is available on:

• GitHub releases
• npm package (@latest dist-tag)

Cheers! 📦🚀