By Daniel Wiessner (Reuters) – Twitter Inc has begun laying off employees under its new owner, Elon Musk. The San Francisco-based social media giant on Friday started laying off staff, expected to number up to 3,700 people – half of its workforce, according to internal plans reviewed by Reuters last week. Twitter is already facing a proposed class action claiming the layoffs are imminent and will violate U.S. and California laws if employees are not given advance notice or severance pay. WHAT DOES U.S. LAW REQUIRE? The federal Worker Adjustment and Retraining Notification (WARN) Act requires b…