NEW YORK — Taijuan Walker has never been better. The right-hander retired 18 consecutive batters in the Mets’ 3-2 win over the Marlins on Saturday at Citi Field. After a first-inning infield single to leadoff hitter Jon Berti, no one reached base against Walker until the seventh inning. In the end, he gave up one run on two hits and recorded nine strikeouts while permitting one walk across 6 2/3 innings and 97 pitches. Walker’s terrific outing lowered his season ERA to 2.88 and earned him his fifth win of the year. It helped that Walker got 11 swings and five whiffs on his slider, which was hi…