feat: expose waitUntil also for serverless runtime & add docs (#14725)

* improvement: expose `waitUntil` also for serverless runtime & add documentation

* changeset

* revert previous changes and add docs

* revert previous changes

* change changeset

---------

Co-authored-by: Tee Ming <chewteeming01@gmail.com>

Author: Fruup

.changeset/slimy-tips-shake.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': minor
+---
+
+chore: mark `RequestContext` as deprecated and refer to `@vercel/functions`

documentation/docs/25-build-and-deploy/90-adapter-vercel.md

@@ -177,6 +177,10 @@ Cookie-based skew protection comes with one caveat: if a user has multiple versi
 
 ## Notes
 
+### Vercel utilities
+
+If you need Vercel-specific utilities like `waitUntil`, use the package [`@vercel/functions`](https://vercel.com/docs/functions/functions-api-reference/vercel-functions-package).
+
 ### Vercel functions
 
 If you have Vercel functions contained in the `api` directory at the project's root, any requests for `/api/*` will _not_ be handled by SvelteKit. You should implement these as [API routes](routing#server) in your SvelteKit app instead, unless you need to use a non-JavaScript language in which case you will need to ensure that you don't have any `/api/*` routes in your SvelteKit app.

packages/adapter-vercel/ambient.d.ts

@@ -1,10 +1,12 @@
-import { RequestContext } from './index.js';
+import type { RequestContext } from './index.js';
 
 declare global {
 	namespace App {
 		export interface Platform {
 			/**
 			 * `context` is only available in Edge Functions
+			 *
+			 * @deprecated Vercel's context is deprecated. Use [`@vercel/functions`](https://vercel.com/docs/functions/functions-api-reference/vercel-functions-package) instead.
 			 */
 			context?: RequestContext;
 		}

packages/adapter-vercel/index.d.ts

@@ -110,6 +110,8 @@ export type Config = (EdgeConfig | ServerlessConfig) & {
 /**
  * An extension to the standard `Request` object that is passed to every Edge Function.
  *
+ * @deprecated - use [`@vercel/functions`](https://vercel.com/docs/functions/functions-api-reference/vercel-functions-package) instead.
+ *
  * @example
  * ```ts
  * import type { RequestContext } from '@vercel/edge';