fix(vercel): allow non-glob cache rules to apply to /

[danielroe]

🔗 Linked issue

resolves #902

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Because of how the Vercel Build Output API works, we can't generate a file-only routing solution for a route rule that is /; in this very specific case we need to add it to the config as well.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[codecov]

Codecov Report

Merging #908 (5a185ab) into main (f7eed65) will decrease coverage by 0.09%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##             main     #908      +/-   ##
==========================================
- Coverage   67.50%   67.42%   -0.09%     
==========================================
  Files          59       59              
  Lines        5866     5878      +12     
  Branches      637      639       +2     
==========================================
+ Hits         3960     3963       +3     
- Misses       1897     1906       +9     
  Partials        9        9              

Impacted Files	Coverage Δ
src/presets/vercel.ts	76.31% <25.00%> (-2.86%)	⬇️

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