Arbitration — Torts — Legal malpractice — Arbitration clause in retainer agreement was unenforceable for violating Rules Regulating the Florida Bar, which prohibit lawyers from making an agreement with a client prospectively providing for mandatory arbitration of fee disputes without advising client in writing that client should consider obtaining independent legal advice — Defendants’ argument that Bar Rule is inapplicable because case does not involve fee dispute does not alter fact that arbitration clause clearly violated rule, which is sufficient to invalidate arbitration clause in its entirety — Severability — Portion of arbitration clause requiring mandatory arbitration of fee disputes was inextricably intertwined with portion requiring arbitration of disputes concerning performance of legal services and is not severable — Trial court erred in dismissing complaint based on arbitration clause