Add Nitro framework boilerplate (#1149)

### Description

Nitro build detects the env var `VERCEL=1` and emits build output API
code, so Nitro already works great on Vercel with zero configuration.
This example is necessary to add the [Nitro framework
entry](vercel/vercel#13472).

### Demo URL

https://nitro-template.vercel.app

### Type of Change

- [X] New Example
- [ ] Example updates (Bug fixes, new features, etc.)
- [ ] Other (changes to the codebase, but not to examples)

### New Example Checklist

- [ ] 🛫 `npm run new-example` was used to create the example
- [X] 📚 The template wasn't used but I carefuly read the [Adding a new
example](https://github.com/vercel/examples#adding-a-new-example) steps
and implemented them in the example
- [X] 📱 Is it responsive? Are mobile and tablets considered?

Author: onsclom

framework-boilerplates/nitro/.gitignore

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+node_modules
+dist
+.data
+.nitro
+.cache
+.output
+.env
+.vercel

framework-boilerplates/nitro/.npmrc

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+shamefully-hoist=true
+strict-peer-dependencies=false

framework-boilerplates/nitro/README.md

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+# Nitro
+
+This directory is a brief example of [Nitro](https://nitro.build/) that can be deployed to Vercel with zero configuration. Go to the [nitro quick start](https://nitro.unjs.io/guide#quick-start) to learn more.
+
+## Deploy Your Own
+
+Deploy your own Nitro project with Vercel.
+
+[![Deploy with Vercel](https://vercel.com/button)](https://vercel.com/new/clone?repository-url=https://github.com/vercel/examples/tree/main/framework-boilerplates/nitro&template=nitro)
+
+_Live Example: https://nitro-template.vercel.app//_
+
+## Developing
+
+Once you've created a project and installed dependencies with `npm install` (or `pnpm install` or `yarn`), start a development server:
+
+```bash
+npm run dev
+```

framework-boilerplates/nitro/nitro.config.ts

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+//https://nitro.unjs.io/config
+export default defineNitroConfig({
+  srcDir: "server"
+});

framework-boilerplates/nitro/package.json

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+{
+  "private": true,
+  "scripts": {
+    "build": "nitro build",
+    "dev": "nitro dev",
+    "prepare": "nitro prepare",
+    "preview": "node .output/server/index.mjs"
+  },
+  "devDependencies": {
+    "nitropack": "latest"
+  }
+}

framework-boilerplates/nitro/server/routes/index.ts

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+export default defineEventHandler((event) => {
+  return "Start by editing <code>server/routes/index.ts</code>.";
+});

framework-boilerplates/nitro/tsconfig.json

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+// https://nitro.unjs.io/guide/typescript
+{
+  "extends": "./.nitro/types/tsconfig.json"
+}