FEATURE-RELEASE: Add Typescript declarations (#42)

This makes it possible to use `@adobe/node-fetch-retry` in Typescript
projects.

It is _also_ used by recent ESLint versions for extra type-checking
goodness, where e.g. missing returns or incorrect return types are
made immediately obvious already while editing code in an IDE.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

index.d.ts

@@ -0,0 +1,20 @@
+import {RequestInfo, RequestInit, Response} from 'node-fetch'
+
+export interface RequestInitWithRetry extends RequestInit {
+  retryOptions?: {
+    retryMaxDuration?: number
+    retryInitialDelay?: number
+    retryBackoff?: number
+    retryOnHttpError?: (error: Error) => boolean
+    retryOnHttpResponse?: (response: Response) => boolean
+    socketTimeout?: number
+    forceSocketTimeout?: boolean
+  }
+}
+
+declare function fetch(
+  url: RequestInfo,
+  init?: RequestInitWithRetry
+): Promise<Response>
+
+export default fetch