adjust migration versioning

src/module/settings.ts

@@ -1,12 +1,12 @@
 export function registerSystemSettings(): void {
     /**
-     * Track the system version upon which point a migration was last applied
+     * Track the migrations version of the latest migration that has been applied
      */
     game.settings.register("ds4", "systemMigrationVersion", {
         name: "System Migration Version",
         scope: "world",
         config: false,
-        type: String,
-        default: "",
+        type: Number,
+        default: -1,
     });
 }