Migrate index.js to use Typescript (#129)

* Migrate index.js to use Typescript

* Fix tests

webapp/src/index.tsx

@@ -0,0 +1,23 @@
+import {Store, Action} from 'redux';
+
+import {GlobalState} from 'mattermost-redux/types/store';
+
+import manifest from './manifest';
+
+// eslint-disable-next-line import/no-unresolved
+import {PluginRegistry} from './types/mattermost-webapp';
+
+export default class Plugin {
+    // eslint-disable-next-line @typescript-eslint/no-unused-vars, @typescript-eslint/no-empty-function
+    public async initialize(registry: PluginRegistry, store: Store<GlobalState, Action<Record<string, unknown>>>) {
+        // @see https://developers.mattermost.com/extend/plugins/webapp/reference/
+    }
+}
+
+declare global {
+    interface Window {
+        registerPlugin(id: string, plugin: Plugin): void
+    }
+}
+
+window.registerPlugin(manifest.id, new Plugin());