WASHINGTON — Pauline Astier stepped to the free-throw line Sunday with 19.3 seconds left in overtime and a chance to pad the Liberty’s two-point lead. It was a big moment for any player, let alone a rookie guard who’s playing in only her second WNBA game. “I was just trying to breathe,” Astier told The Post. “I was like, ‘OK, it’s just more free throws.’ … I was just trying to be cool.” Astier made both free throws on that trip to the line and sank two more later to help the Liberty clinch a 98-93 win against the Washington Mystics . Astier has been with New York for about two weeks, but she’s already impressed the team’s stars with her confidence, poise and fearlessness while stepping in for an injured Sabrina Ionescu. Pauline Astier attempts a shot during the Liberty’s May 10 win. NBAE via Getty Images Jonquel Jones, who watched overtime from the bench, felt Astier’s performance down the stretch was both awe inspiring and anxiety inducing. “She was out there splitting double-teams and stuff,” Jones said.…