feat(vercel): enable streaming support out of the box

[pi0]

🔗 Linked issue

resolves #1505

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 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)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR improve js docs for vercel sls config and also enables supportsResponseStreaming flag by default (can be disabled by user).

Now that we are using a h3 version that is compatible with streaming, we can safely enable streaming. I have requested vercel team to verify this.

📝 Checklist

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

[codecov]

Codecov Report

Merging #1514 (7734365) into main (e8e806a) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1514      +/-   ##
==========================================
+ Coverage   76.25%   76.26%   +0.01%     
==========================================
  Files          73       73              
  Lines        7462     7466       +4     
  Branches      732      732              
==========================================
+ Hits         5690     5694       +4     
  Misses       1771     1771              
  Partials        1        1              

Files Changed	Coverage Δ
src/presets/vercel.ts	92.90% <100.00%> (+0.09%)	⬆️