Mark _serviceBrand properties using declare (#99312)

_serviceBrand is only used for typing and should not result in emit

Also adds readonly

src/vs/workbench/services/userDataSync/electron-browser/userDataSyncService.ts

@@ -14,7 +14,7 @@ import { URI } from 'vs/base/common/uri';
 
 export class UserDataSyncService extends Disposable implements IUserDataSyncService {
 
-	_serviceBrand: undefined;
+	declare readonly _serviceBrand: undefined;
 
 	private readonly channel: IChannel;