“We conclude that the District Court judge erred in holding that G. L. c. 218, § 19A, constrains a court from looking beyond a plaintiff’s initial statement of damages in assessing whether there is a reasonable likelihood that recovery by the plaintiff will exceed $50,000. Rather, the statute requires the court to consider the nature of the action itself—and thus the complaint then before the court,” Associate Justice Elizabeth N. Dewar wrote on behalf of the SJC.