Three weeks after its first coronavirus infection was discovered, the New York City region reached an alarming milestone on Sunday: It now accounts for roughly 5 percent of the world’s confirmed cases, making it an epicenter of the pandemic and increasing pressure on officials to take more drastic measures. Moving to stem the crisis on multiple fronts, Gov. Andrew M. Cuomo of New York pleaded with federal officials to nationalize the manufacturing of medical supplies and ordered New York City to crack down on people congregating in public. He suggested some streets could be closed, allowing pedestrians more space.