convert macOS check to utility function

src/utils/os.test.ts

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2021 The Pybricks Authors
+
+import { isMacOS } from './os';
+
+describe('isMacOS', () => {
+    test('is true', () => {
+        jest.spyOn(navigator, 'platform', 'get').mockReturnValue('MacIntel');
+        expect(isMacOS()).toBeTruthy();
+    });
+    test('is false', () => {
+        jest.spyOn(navigator, 'platform', 'get').mockReturnValue('Win32');
+        expect(isMacOS()).toBeFalsy();
+    });
+});

src/utils/os.ts

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2021 The Pybricks Authors
+
+// Utility functions for dealing with operating systems.
+
+export function isMacOS(): boolean {
+    return /mac/i.test(navigator.platform);
+}