feat: add event.method and event.headers

[Hebilicious]

🔗 Linked issue

#423, #73

❓ 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 adds event.method and event.headers support.

📝 Checklist

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

[codecov]

Codecov Report

Merging #429 (2361c85) into main (fb9e3e3) will decrease coverage by 1.66%.
The diff coverage is 100.00%.

❗ Current head 2361c85 differs from pull request most recent head 80aed9e. Consider uploading reports for the commit 80aed9e to get more accurate results

@@            Coverage Diff             @@
##             main     #429      +/-   ##
==========================================
- Coverage   76.82%   75.16%   -1.66%     
==========================================
  Files          26       26              
  Lines        2554     2569      +15     
  Branches      367      369       +2     
==========================================
- Hits         1962     1931      -31     
- Misses        592      638      +46     

Impacted Files	Coverage Δ
src/event/event.ts	57.75% <100.00%> (+6.27%)	⬆️

... and 3 files with indirect coverage changes

[pi0]

Thanks for the PR!

I've made a couple of updates to use a getter with lazy evaluation and keep the public interface read-only. Similar to _handled we have an internal way to override them using event._method and event._headers -- Not to be advised --

Fix tests for Node 16 not having global Headers. In H3 we have a working polyfill but thinking to node depend on for best web compatible behavior.