[edge] Add RequestContext type (#9526)

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

packages/edge/src/index.ts

@@ -4,5 +4,6 @@ export * from './middleware-helpers';
 export type { Geo } from './edge-headers';
 export * from './edge-headers';
 export * from './response';
+export type { RequestContext } from './request';
 
 import './published-types.d.ts';

packages/edge/src/request.ts

@@ -0,0 +1,52 @@
+/**
+ * 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
+ * }
+ * ```
+ */
+export interface RequestContext {
+  /**
+   * 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 });
+   *  }
+   * }
+   * ```
+   */
+  waitUntil(
+    /**
+     * A promise that will be kept alive until it resolves or rejects.
+     */ promise: Promise<unknown>
+  ): void;
+}