unipicker: add sigmanificient to maintainers

pkgs/applications/misc/unipicker/default.nix

···
     description = "A CLI utility for searching unicode characters by description and optionally copying them to clipboard";
     homepage = "https://github.com/jeremija/unipicker";
     license = licenses.mit;
-    maintainers = with maintainers; [ ];
+    maintainers = with maintainers; [ sigmanificient ];
     platforms = platforms.unix;
     mainProgram = "unipicker";
    };