feat: plain and web adapters

[pi0]

🔗 Linked issue

Resolves #458

❓ 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 two new plain and web adapters to use h3 directly outside of Node.js environments!

Until now, it was only possible to use h3 in workers and lambda environments by either using Nitropack or custom code. With two new adapters, integration will be much more straightforward.

Two new utils depend on unenv framework to keep backward compatibility with Node.js API. This will be fully optional in the next major version of H3 once all h3 internal utils were decoupled.

---

Worker test deployment (deno deploy): https://rich-owl-49.deno.dev

import { createApp, eventHandler, toWebHandler } from "h3";

const app = createApp();

app.use(
  eventHandler((event) => ({
    path: event.path,
    message: "H3 works on Workers!",
  }))
);

Deno.serve(toWebHandler(app));

---

Total bundle size analysis (with esbuild):

Node: 19.4kb
Node min: 8.6kb
Node min gzip: 4.0kb

Web: 42kb (+22.6 kb)
Web min: 19.6kb (+11 kb)
Web min gzip: 8.1kb (+4.1 kb)

📝 Checklist

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

[codecov]

Codecov Report

Merging #483 (cf33cd5) into main (c266755) will increase coverage by 0.23%.
Report is 2 commits behind head on main.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##             main     #483      +/-   ##
==========================================
+ Coverage   82.56%   82.79%   +0.23%     
==========================================
  Files          28       31       +3     
  Lines        3234     3424     +190     
  Branches      494      515      +21     
==========================================
+ Hits         2670     2835     +165     
- Misses        564      589      +25     

Files Changed	Coverage Δ
src/adapters/plain.ts	87.12% <87.12%> (ø)
src/adapters/index.ts	100.00% <100.00%> (ø)
src/adapters/node.ts	85.08% <100.00%> (ø)
src/adapters/web.ts	100.00% <100.00%> (ø)
src/app.ts	96.70% <100.00%> (-0.73%)	⬇️
src/event/event.ts	90.36% <100.00%> (ø)
src/index.ts	100.00% <100.00%> (ø)
src/utils/cookie.ts	93.78% <100.00%> (+0.79%)	⬆️

... and 1 file with indirect coverage changes

[pi0]

Let's try it!

[magne4000]

What does this actually mean? I did not find any other information regarding this comment.
Does this mean that internal representation as IncomingMessage/ServerResponse will change at some point? If yes, is there discussions somewhere on the subject? If no, what was the idea?

For the context, I'm experimenting on generic handlers (and middlewares), and currently listing how each popular server implements handlers and middlewares.

[magne4000]

Some more context on a dedicated repo https://github.com/magne4000/universal-handler