Make waitUntil consistent for Node.js & Edge (#11553)

This PR makes `waitUntil` consistent when interacting with it via `vc
dev`. That includes:

**be possible to call `waitUntil` from the context second argument in
web handlers functions (Node.js & Edge)**:

```js
export function GET(request, { waitUntil }) {
  waitUntil(fetch('https://vercel.com'));
  return new Response('OK');
}
```

**be possible to call `waitUntil` imported from `@vercel/functions`**
(Node.js & Edge):

```js
import { waitUntil } from '@vercel/functions';

export function GET(request) {
  waitUntil(fetch('https://vercel.com'));
  return new Response('OK');
}
```

Additionally, `@vercel/functions` can be adopted for other framework to
take advantage of this pattern at Vercel.

---------

Co-authored-by: Javi Velasco <javier.velasco86@gmail.com>

packages/node/test/dev-fixtures/wait-until-ctx-node.js

@@ -0,0 +1,10 @@
+const baseUrl = ({ headers }) =>
+  `${headers.get('x-forwarded-proto')}://${headers.get('x-forwarded-host')}`;
+
+export function GET(request, ctx) {
+  const { searchParams } = new URL(request.url, baseUrl(request));
+  const url = searchParams.get('url');
+
+  ctx.waitUntil(fetch(url));
+  return Response.json({ keys: Object.keys(ctx) });
+}