feat: support universal { fetch } handlers

[pi0]

H3 by default supports (event: H3Event) => any handler format.

For better external compatibility, we have introduced app.mount(<base>, <fetchable>) where it is possible to mount an external framework like Elysia or Hono, or simply use { fetch(req: Request) => Response } universal alternative format.

This PR adds Fetchable object format support for also h3App[method]("route", <fetchable>) and defineHandler.

import { H3 } from "h3";

const app = new H3();

app.get("/hello/:id", {
  fetch: (req) => new Response("Standards FTW!"),
});

export default app;

This is particularly useful to be used together with filesystem routing like Nitro to mount any standard fetchable handler, also via filesystem routes.

// routes/hello/[id].ts

// Standard handler
import { defineHandler } from "h3"
export default defineHandler({
  fetch: (req) => new Response("Standards FTW!"),
})

// Hono (https://hono.dev/)
import { Hono } from "hono";
export default new Hono().get("/", (c) =>
   c.text("Hello from server entry (Hono)!")
 );

// Elysia (https://elysiajs.com/)
import { Elysia } from "elysia";
export default new Elysia().get("/", () => "Hello from server entry (Elysia)!");

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/handler.ts	88.88%	2 Missing ⚠️

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