fix(prerender): free up memory after each route is written to the disk

[pi0]

🔗 Linked issue

Resolves #1535

❓ 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 resolves the high memory usage issue when prerendering lots of routes by cleaning up the memory of route contents as soon as being written to the disk.

Since we were already typing the route.data and route.contents as optional, as a non breaking change they are only available (and can be set) before disk write. We could additionally support reading back contents from disk when accessing .data and .contents getters using fs sync methods but I think we can do it only if necessary.

Testing on reproduction in #1535 there seems no memory leaks anymore generating all 100k routes.

📝 Checklist

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

[codecov]

Codecov Report

Merging #1536 (be5b7b8) into main (451d314) will decrease coverage by 0.10%.
The diff coverage is 57.69%.

@@            Coverage Diff             @@
##             main    #1536      +/-   ##
==========================================
- Coverage   76.25%   76.15%   -0.10%     
==========================================
  Files          73       73              
  Lines        7478     7494      +16     
  Branches      734      733       -1     
==========================================
+ Hits         5702     5707       +5     
- Misses       1775     1786      +11     
  Partials        1        1              

Files Changed	Coverage Δ
src/prerender.ts	84.81% <57.69%> (-2.27%)	⬇️