Migrate app to TypeScript (#1637)

* Initial setup and migrated src/common

* WIP

* WIP

* WIP

* Main module basically finished

* Renderer process migrated

* Added CI step and some fixes

* Fixed remainder of issues and added proper ESLint config

* Fixed a couple issues

* Progress!

* Some more fixes

* Fixed a test

* Fix build step

* PR feedback

src/types/window.ts

@@ -0,0 +1,27 @@
+// Copyright (c) 2016-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import {ipcRenderer} from 'electron/renderer';
+
+declare global {
+    interface Window {
+        ipcRenderer: {
+            send: typeof ipcRenderer.send;
+            on: (channel: string, listener: (...args: any[]) => void) => void;
+            invoke: typeof ipcRenderer.invoke;
+        };
+        os: {
+            isWindows10: boolean;
+        };
+        process: {
+            platform: NodeJS.Platform;
+            env: {
+                user?: string;
+                username?: string;
+            };
+        };
+        timers: {
+            setImmediate: typeof setImmediate;
+        };
+    }
+}