[edge] Add RequestContext type (#9526)

Co-authored-by: Kiko Beats <josefrancisco.verdu@gmail.com>

packages/edge/docs/interfaces/RequestContext.md

@@ -0,0 +1,79 @@
+# Interface: RequestContext
+
+An extension to the standard `Request` object that is passed to every Edge Function.
+
+**`Example`**
+
+```ts
+import type { RequestContext } from '@vercel/edge';
+
+export default async function handler(
+  request: Request,
+  ctx: RequestContext
+): Promise<Response> {
+  // ctx is the RequestContext
+}
+```
+
+## Table of contents
+
+### Methods
+
+- [waitUntil](RequestContext.md#waituntil)
+
+## Methods
+
+### waitUntil
+
+▸ **waitUntil**(`promise`): `void`
+
+A method that can be used to keep the function running after a response has been sent.
+This is useful when you have an async task that you want to keep running even after the
+response has been sent and the request has ended.
+
+**`Example`**
+
+<caption>Sending an internal error to an error tracking service</caption>
+
+```ts
+import type { RequestContext } from '@vercel/edge';
+
+export async function handleRequest(
+  request: Request,
+  ctx: RequestContext
+): Promise<Response> {
+  try {
+    return await myFunctionThatReturnsResponse();
+  } catch (e) {
+    ctx.waitUntil(
+      (async () => {
+        // report this error to your error tracking service
+        await fetch('https://my-error-tracking-service.com', {
+          method: 'POST',
+          body: JSON.stringify({
+            stack: e.stack,
+            message: e.message,
+            name: e.name,
+            url: request.url,
+          }),
+        });
+      })()
+    );
+    return new Response('Internal Server Error', { status: 500 });
+  }
+}
+```
+
+#### Parameters
+
+| Name      | Type                                                                                                              | Description                                                     |
+| :-------- | :---------------------------------------------------------------------------------------------------------------- | :-------------------------------------------------------------- |
+| `promise` | [`Promise`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise)<`unknown`\> | A promise that will be kept alive until it resolves or rejects. |
+
+#### Returns
+
+`void`
+
+#### Defined in
+
+[packages/edge/src/request.ts:47](https://github.com/vercel/vercel/blob/main/packages/edge/src/request.ts#L47)