Port multiloader build to Fabric and Forge 1.20.1

migrate-to-multiloader.sh

@@ -8,6 +8,11 @@
 set -euo pipefail
 cd "$(dirname "$0")"
 
+if [[ ! -e src/main/java/xyz/nightsync/skinsg/SkinsgConfig.java ]]; then
+  echo "Migration already applied; src/ is not present."
+  exit 0
+fi
+
 CJ=common/src/main/java/xyz/nightsync/skinsg
 CR=common/src/main/resources
 mkdir -p "$CJ" "$CR" neoforge/src/main/resources/META-INF