FKA Twigs is opening up about her decision to come forward with abuse allegations against ex Shia LaBeouf. The British “cellophane” singer, 34, told GQ that the tranquility she projected at the time she filed a lawsuit against the “Disturbia” star, 36, for sexual battery in December 2020 is among her “greatest achievements.” “One of the greatest achievements of the whole of my life was keeping my s— together,” said the Grammy nominee, born Tahliah Debrett Barnett. “It was one of the things that I’m most proud of, that I was able to go on tour and do interviews and stay graceful and keep that c…