Prefer type to interface and add an ESLint rule

ts/textsecure/WebAPI.ts

@@ -1,4 +1,4 @@
-// Copyright 2020 Signal Messenger, LLC
+// Copyright 2020-2021 Signal Messenger, LLC
 // SPDX-License-Identifier: AGPL-3.0-only
 
 /* eslint-disable no-param-reassign */
@@ -586,6 +586,8 @@ async function _outerAjax(url: string | null, options: PromiseAjaxOptionsType) {
 }
 
 declare global {
+  // We want to extend `Error`, so we need an interface.
+  // eslint-disable-next-line no-restricted-syntax
   interface Error {
     code?: number | string;
     response?: any;