add app install button

src/utils/dom.ts

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2021 The Pybricks Authors
+
+// Chromium-only API
+// https://developer.mozilla.org/en-US/docs/Web/API/BeforeInstallPromptEvent
+export interface BeforeInstallPromptEvent extends Event {
+    readonly platforms: string[];
+    readonly userChoice: Promise<{
+        outcome: 'accepted' | 'dismissed';
+        platform: string;
+    }>;
+    prompt(): Promise<void>;
+}