The multi-talented David Sabella is a performer who walks his talk. He's the author (with Sue Matsuki) of "So You Want To Sing Cabaret," a comprehensive study of the genre, and clearly Sabella knows whereof he speaks"and sings. His post-pandemic show at Pangea, Pandemic Relief, was the marriage of technical perfection with artistic excellence, authenticity and a lot of heart, not to mention Sabella's clear-as-a-bell …