It’s time for Americans to use the word “oligarch” to describe our own billionaires, not just those in Russia. Because it clarifies things. It helps explain what’s happening at Liberty State Park as Reebok billionaire Paul Fireman paves the way to grab a pristine corner of the park for himself. He wants to expand his members-only golf course, whether we mere citizens like it or not. And the Legislature, at every turn, has rejected pleas to protect the land at stake, a 22-acre spit of gorgeous oceanfront, including a half-mile of beach, the biggest stretch in New York Harbor. Known as Caven’s P…