MUMBAI/NEW DELHI (Reuters) - Jet Airways Ltd shares plunged on Thursday, battered by fears about whether the grounded Indian airline would fly again, even as carriers like rival SpiceJet rushed in to plug a supply gap in the wake of their rival's demise.Once India's largest private airline, Jet halted all flight operations indefinitely on Wednesday evening after lenders led by State Bank of India declined to extend more funds to keep the carrier going. Investors punished Jet shares on Thursday,..