[@types/node-fetch] Fix FetchError class definition (#40581)

* Fix FetchError class definition

* Add myself to "Definitions by" section of package header

types/node-fetch/index.d.ts

@@ -9,6 +9,7 @@
 //                 Brandon Wilson <https://github.com/wilsonianb>
 //                 Steve Faulkner <https://github.com/southpolesteve>
 //                 ExE Boss <https://github.com/ExE-Boss>
+//                 Alex Savin <https://github.com/alexandrusavin>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 /// <reference types="node" />
@@ -148,9 +149,13 @@ export class Body {
     timeout: number;
 }
 
+interface SystemError extends Error {
+    code?: string;
+}
+
 export class FetchError extends Error {
     name: "FetchError";
-    constructor(message: string, type: string, systemError?: string);
+    constructor(message: string, type: string, systemError?: SystemError);
     type: string;
     code?: string;
     errno?: string;

types/node-fetch/node-fetch-tests.ts

@@ -158,7 +158,15 @@ function test_isRedirect() {
 }
 
 function test_FetchError() {
-    new FetchError("message", "type", "systemError");
+    new FetchError("message", "type", {
+        name: 'Error',
+        message: 'Error message',
+        code: "systemError",
+    });
+    new FetchError("message", "type", {
+        name: 'Error',
+        message: "Error without code",
+    });
     new FetchError("message", "type");
 }