feat!: opt-in using event format using defineEventHandler

[pi0]

A bit of context: #73

This refactor, is starting point to move away from (req, res) signature to (event) format with utilities with minimum possible changes to both core and usage. event is still a simple function { req, res }.

Tests are untouched and handlers will be automatically upgraded with a wrapper to event format, unless directly opting in an using defineEventHandler. Still using a semver-minor to alert about potential behavior changes.

// Keeps working
app.use((req, res) => 'Hello World!')

// Opt-in to new event format
import { defineEventHandler } from 'h3'

app.use(defineEventHandler(() => 'Hello World'))