Create DB version of Against; add passthroughs

SabreTools.DatFiles/ItemMappings.cs

@@ -0,0 +1,12 @@
+namespace SabreTools.DatFiles
+{
+    /// <summary>
+    /// Class used during deduplication
+    /// </summary>
+    public struct ItemMappings(DatItems.DatItem item, long machineId, long sourceId)
+    {
+        public DatItems.DatItem Item = item;
+        public long MachineId = machineId;
+        public long SourceId = sourceId;
+    }
+}