feat(platform-utils): Get and react to changes to the system theme (#161)

These changes will allow the WebExtension (and later the desktop application) to respect the system theme.

I've added the Electron implementation until I realized that the required API [has been implemented but not released yet](https://www.electronjs.org/docs/api/native-theme/history). Let me know if you I should remove the code.

Part of https://github.com/bitwarden/browser/issues/1256.

https://www.electronjs.org/docs/api/native-theme

Co-authored-by: Chad Scharf <3904944+cscharf@users.noreply.github.com>

src/cli/services/cliPlatformUtils.service.ts

@@ -146,6 +146,13 @@ export class CliPlatformUtilsService implements PlatformUtilsService {
         return Promise.resolve(false);
     }
 
+    getDefaultSystemTheme() {
+        return 'light' as 'light' | 'dark';
+    }
+
+    onDefaultSystemThemeChange() {
+    }
+
     supportsSecureStorage(): boolean {
         return false;
     }